I have an windows XP pc. I noticed if i leave the PC on for a few days, it catches some viruses. is this possible and is it better to shut it down when not being used.

Do i need any security software for viruses and trojans and firewalls anda registr cleaner software.
or would the windows firewall and combofix and hijackthis and malaware would do the job free of charge.

what do you usually recommend.
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I would NOT recommend using such tools as combofix or hijackthis for proactive virus protection. These particular tools are for use AFTER an infection. What you need is a set of tools for the prevention of infection.

My first recommendation would be the use of a router to act as a hardware firewall if one is not already in place. If you are ONLY using the modem provided with a cable connection, you probably don't have a built-in router. Most DSL connections are provided through a modem which contains a router. Check your IP address to verify the existence of a router on your connection. If the IP address for your workstation begins with 192.168... or 10.1.10... you are behind a router already and do not need additional HARDWARE firewall protection.

Windows XP provides a software firewall (Windows Firewall) which may also be activated.

I would NOT recommend a registry cleaner. There are mixed reviews regarding these programs. Are they smart enough to correctly edit the registry? Remember that the improper editing of your registry can result in the crashing of your system and necessitate the reinstallation of Windows.

Antivirus software is also necessary. The concept that "free is good enough" is only acceptable if your data and your time are not worth much. Consider spending the money to properly protect yourself. I recommend a corporate (not consumer) grade antivirus program. My personal preference is Symantec's Norton Antivirus (SAV) or Symantec's Endpoint (SEP).

SAV is a mature and proven product. SEP is relatively new and is still going through frequent version revisions. SAV contains no additional firewall. SEP, on the other hand, is an Internet security package and includes "Network Intrusion Protection" - in effect: a firewall. Neither product seems to weigh down the performance of the PC and both appear to be quite effective.

In response to the need to shut down your system when not in use: MY systems stay on all the time. The monitors are shut off when not in use (they use more power than the computer itself) but the systems are on (and available) all the time. I rarely have issues with viruses or failing systems. They are on a cable connection, behind a router, and have been running various versions SAV for several years. About a month ago, I switched 3 of the systems to SEP - they've been fine.
Robert PraschCommented:
I have been running my computer with a free anti-virus program named AVG at www.avg.com, now this program is good at detecting most spyware and viruses and scans website links and emails for you. This is all done for free and uses limited resources.

Keep in mind though that the free version has no enhanced firewall and the windows firewall is not that effective, if you would like more security you may purchase the pro version of AVG 9.0, Business professionals usually use Symantec Anti-Virus, Mcafee, or something of the sort. For personal use you can just stick with AVG.

Now it is true that if you leave your computer on you are at risk of being attacked, if someone is trying to get into a network to plant a trojan or virus, keeping it turned on makes it easier for them to accomplish this.
Any time you have an 'always on internet connection' you are susceptible to intrusion.  Windows firewall is better than nothing, but it's not a complete solution.  Depending on your service provider you can have a fair level of protection through their firewalls.  If you modem/router for your internet connection does not have a firewall built in, I would highly recommend getting a firewall/router.  They can be purchased fairly inexpensively from most any store that sells computers, are set to block all bad stuff out of the box, and are not that difficult to setup if the instructions are followed.   I would also highly recommend an anti-virus solution to the package.  I have used AVG on many different systems with good success.  
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Use following

1. Malewarebytes >>          http:www.malwarebytes.org
2. Superantispyware >>     http://www.superantispyware.com/
3. Hitmanpro's Surfright >> http://www.surfright.nl/en/hitmanpro
4. Hijack this >>                   http://download.cnet.com/Trend-Micro-HijackThis/3000-8022_4-10227353.html
5. Combofix >>                    http://www.bleepingcomputer.com/combofix/how-to-use-combofix
6. Bitdefender >>                http://kb.bitdefender.com/site/KnowledgeBase/consumer/#627
7. SPybot                            http://www.filehippo.com/download_spybot_search_destroy/
8. CCleaner                         http://www.filehippo.com/download_ccleaner/
9. Registry Cleaner             http://www.eusing.com/free_registry_cleaner/registry_cleaner.htm
10. Defraggler                     http://www.filehippo.com/download_defraggler/

Antivirus (Use any one of them)
1. Eset Nod32
2.Symentac endpoint
3.Norton ghost

Hope this will help you
I agree with rigador, DO NOT use Combofix as it can brick a machine when used for anything but a specific purpose.

Some free anti-virus programs that provide real time protection that a lot of people use and love would be Avast! anti-virus and Avira.  

If you currently have an infection, please run Malwarebytes (malwarebytes.org) as its been the most successful, but if it doesn't catch everything - SuperAntiSpyware has worked well for me in the past too.  If these or your AV has trouble connecting to the internet, please check in Internet Explorer -> Tools -> Options - > Connections Tab -> LAN Settings -> ensure that "use a proxy server" is NOT checked.
Remember that free anti-virus programs will not offer real time protection.  I would suggest any one to get real time protection.  After all, an ounce of prevention is worth a pound of cure!  Superantispyware is having a sale right now and you can get the real time protection for $9.99.  I installed it on all of my PC's.   Great software at a great price.
1. You need to have an antivirus protection. Free one is not bad, I'm using the FREE Avast.
2. You need to have a Firewall, I'm just using the Windows Firewall.
3. You also need to have an anti-malware with real-time protection(not free but cheap).
     MalwareBytes seems to be the best so far out there and it's only cheap for a lifetime
You don't need a Registry cleaner.....but as part of basic cleanup a temp file cleaner is good like TFC Cleaner, ATF Cleaner, or CCleaner.
As already mentioned ComboFix and HijackThis etc are removal tools they can't protect the system as they don't have real-time protection, they are just removal tools.
ComboFix should not be used without guidance from a Helper who knows how to use the tool. The tool was created for the Helpers.
In addition to the other great suggestions posted; if they all fail, try creating a bootable antivirus CD. If that doesn't fix it, then you've got some serious problems. It's always good to keep on hand at anytime:
What I like is that there are just some pesky items that can't be removed while in Windows. I run from a bootable source first, then go into Windows and see what's left over and then deal with it after. The bootable CD sometimes will take care of 80-100% of the infected items; making it that much easier. Best of luck to you.
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