I had a system checked in with a number of viruses and sporadic issues, but one caused me quite a few headaches. After the system was booted, it would run fine for a few minutes, but then it would gradually slow, eventually becoming unusable. I finally found found (in task manager) that svchost.exe was running high. I could kill it off, but it would return within a minute, and use high memory (800,000K+).
After running Process Explorer from Sysinternals and a few other apps to try and diagnose the issue, I opted to run Kaspersky’s TDSSKiller. Surely enough, it found a Rootkit on the system. After a reboot, it was running fine.
Need to download TDSSKiller? It is free. Just visit http://support.kaspersky.com/faq/?qid=208283363
I hope this helps!