constant disk usage(suspected virus)

I use the malware removal guide above as my bible on removing viruses and has worked for years, but I think one's slipped through my antivirus software... I'm running windows vista and last time i knew(windows xp that is) it isn't normal for your disk to be in constant use, so i checked it out and apparently svchost.exe (LocalSystemNetworkRestricted) in the cause, and using 100+ mb of memory. I can't find any viruses, spyware etc with the scanners though. I've run out of ideas, could someone help me?