Perform multiple anti virus scans using several products - see
Slow Internet and Vistax32 after a while... - Vista Forums for a good list of products to use.
Also, get Malwarebytes' Antimalware and run it as well. I have a feeling that you have been infected by something for a long time and it is now time-bombing your system since after the upgrade....
Also, please fill in your system specs underneath your nickname on the left - there is a link that says "My system specs" - click on that and you'll see an additional link that says "Update your system specs" - click that.
Please be as specific as possible - for example, not just Windows 7, but build and architecture (mine is Windows 7 Ultimate x64 RTM (RTM is build 7600)