A very useful site that may be worth a try is
www.sysinternals.com
It is a Microsoft TechNet site which we use a lot, however you dont need to be signed in, so if you havent got a TechNet account you can still access these tools.
You can then look at the Process Utilities and see what little nasties are running (even if they are not showing up in Task Manager etc). If there is a small app (virus, spyware, adware etc) this will list it. You may need to filter the list down as you will see how many millions of processes are actually running every minute!
Hope this helps.
J