If you can get windows to open, do a ctrl+alt+delete. Then look in the processes, at the bottom it will tell you how much cpu is being used. If it is going up and down from 50% to almost 100% look in the processes to see which one is using up the most. Then write it down as this is probably the culprit.

This should help you narrow it down to either a hardware or software problem. If it is hardware, you might need more of a techie to look at it, and in some cases even if it is a software problem ie: viruses etc.
Sometimes it is very difficult to get the viruses, or trojans off of a computer.

I have found that if once a year I reinstall windows, doing a complete format and install, it keeps my computer in tip top shape. This is if all the hardware is in good shape. Hard to say what it is without sitting right there, but I hope this helps you narrow it down.