I need a software to detect all devices connected to my network including printers, routers, Switches, VOIP phones and of course pc's?

Take a look at nmap

http://nmap.org/book/man.html

For a freeware graphical solution you can also check out Look@LAN
http://www.lookatlan.com/

As previously suggested - nmap will do it.  It's free and excellent.

The command you'll probably want to use is:

nmap -O 192.168.0.0/24

or whatever your address range is - the O switch does OS detection.

P

Yes nmap is the tool. If you are on a windows box and you don't want to throw the data into a script, then the DotNmap gui front end may be handy too.

I started checking some of these out. I think I am looking for something like LAN Surveyor.
I will install the demo to see if it is what I need. But it is a little pricey for my network of 30pcs, 2 servers and 30 voip phones.

I Use advancedport scanner al the time
http://www.radmin.com/products/utilities/portscanner.php
its really good, and fairly smart, easy to use.

I would also setup a linux server (great tools there).  Use arpwatch.  This watches for arp packets.  While the key for this software is for monitoring in case a pc comes online that takes over another's IP or possibly a MITM attack, it also sees any computer on a segment.

This could be important because the other softwares will actively find other hosts, arpwatch passively finds them.  The difference is that if you have one network segment, but people are running a rogue computer with an IP not matching your scheme you won't detect them.  You want to find these as well since they can usually only cause problems.

Just a suggestion anyway.

I tried Lansurveyor and I like it but it is too expensive for now. I will try to convince my client to buy it. We are sticking to Spiceworks which is not as accurate as lansurveyor but works.

Thanks