Download SpeedFan from http://www.download.com and check your temperatures. If the cores are under 50 degrees C, you're OK, if over 65, you have a problem, in between bears watching.
Get your antivirus program up to date and boot into Safe Mode. Note that some viruses can hide themselves from your antivirus program in normal mode, so you really need to scan in Safe Mode. To get into Safe Mode when you first power up, hit F8 about once a second until you get the menu and select Safe Mode. Then run a full system scan.
You can check for corrupted system files. Open an administrator command prompt and run SFC if the above doesn't help. Click START, then type CMD in the search box, right-click CMD.EXE and click Run as administrator. Then from the command prompt type sfc /scannow.
Finally if all else fails, you can check the rather cryptic system event log. To do that click Start -> Control Panel -> Administrative Tools -> Event Viewer. Once in Event Viewer click on the system log and scroll through the entries looking for those flagged "error" to see if you can find hints as to where the problem could be.
When you get your system back in good working order, I urge you to back your system up to an external hard drive and make regular periodic updates to it.
I hope this helps. Good luck.