Internet problem (virus)

i have 3 computers all attached to a router all able to connect to the internet. But on one computer i can access the internet for a while and then i lose connection, i can still open internet explorer, avant ect and i can still access the network ive tryed reseting my network card (both wireless and wired) and im having no joy. When this happens i get an error from command prompt when i try and open that and also from task manager when i try to launch it from the tray. It looks like a virus but im not sure which one ive scanned with every virus detector and anti rootkit detector i can get my hands on and nothing so far. Any Ideas?

p.s. this computer is running vista premium legit copy.

Many Thanks crackruckles