Aye try the de-frag and see if that helps. Would also recomend clearing your start-up programs to test it to run this:
Goto Start then Run
Type MSCONFIG and press enter
Then goto the Startup tab and disable all but re-enable security centre
Then goto Services, tick hide all microsoft services
And again disable all but re-enable your security.
Failing that it still sounds like Spyware to me even though you did a format it is very easy to get spyware and as you said it most happens when your online as in on a web browser. I would update your Security Centre to Avast and your Spyware protection to SuperAntiSpyware, just do a search in google for both of them they are both free.
If the problem still persists then I would put the system into Safe Mode with Networking and test there, to do this:
When the system is starting up tap the F8 key
Choose Safe Mode With Networkign from the advanced options
Login to windows
This mode with have distorted graphics etc but not to worry its for testing purposes so all your drivers etc arent loaded. If the problem is still hapening within this then I say you would have a hardware fault. More than likely would be your hard drive at this point or maybe a piece of dodgy memory. If it doesnt bother you too much then just keep using the system until the part completely fails but if its annoys you then strip the system and replace the hard drive and test if the problem is still there then try replacing some memory.
Hope this helps. And if you want I can connect to your system and have a look for you
Christy