How to find source of system bottle neck or lag

I have just cleaned a laptop that had the fbi moneypak virus
I scanned with a few other proggies and nothing was found however the system is extremely laggy
Task manager reports low usage and the system is an i7 with 2gb ram

So since there are no viruses or malware how do i locate the problem

Try this, see if it helps id the problem: How to perform a clean boot to troubleshoot a problem in Windows Vista, Windows 7, or Windows 8

We have many fine tutorials here, written by some very knowledgeable people, try these and see if they help.

http://www.sevenforums.com/tutorials/472-troubleshooting.html

http://www.sevenforums.com/tutorials/220165-troubleshooting-steps-windows-7-a.html

If these don't locate/solve your problem, you could run a sfc/scannow to check for and possibly repair any system files the virus messed up.

http://www.sevenforums.com/tutorials/1538-sfc-scannow-command-system-file-checker.html

If none of these solve your problem , let us know and we'll go from there. Hope these help