Issue connecting to the internet

We have had this problem on and off for the past couple weeks. We are able to get to some sites and not to others. Seems to be mostly on Windows XP systems. Again, some systems seem to have more trouble than others. All of our Windows 7 systems are fine. No problems getting to the internet and viewing all sites. We have a Cisco router from CableVision, WatchGuard 750e XTM appliance and two Asante 48 port Switches. We have rebooted all of these a couple of times and it seems to help. Everybody seems to be working fine and then a day or so goes by and people start to call that they cannot get to some sites. This happens with all browsers. Any ideas would be much appreciated.
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SStoryConnect With a Mentor Commented:
You need to investigate the computers that can't get to those sites.
1.) Can they get to anything?
     a.) From a command prompt do  
           nslookup www.whatever.com (change to the site in question)

           If you don't get a valid lookup, that is the problem.  If you do get a valid lookup,
           try hitting that site using the IP address given from nslookup.
           Example:  http://65.15.27.35   (this is a random IP that chose)

      b.) From a command prompt do
            tracert www.whatever.com (change to the site in question)
            see if it fails before getting there.  I've had that with an ISP before where their    
            upstream routers were on the blitz

2.) Do they have malware or is the browser hijacked?

3.) Is it a content delivery system or does it contain links to outside sites that are blocked by the firewall?

4.) Is the C:\Windows\System32\drivers\etc\hosts redirecting from what you expect to somewhere else?

5.) Are these sites down for everyone or just you?
Check that here: http://www.downforeveryoneorjustme.com/
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