lets try something simple.

switch the PC off. disconnect all but the keyboard and mouse.

then do the following

1. Restart your computer.
2. When the machine first starts again it will generally list some equipment that is installed in your machine, amount of memory, hard drives installed etc. At this point you should gently tap the F8 key repeatedly until you are presented with a Windows XP Advanced Options menu.
3. Select the option for Safe Mode using the arrow keys.
4. Then press enter on your keyboard to boot into Safe Mode.

run in safe mode for about 1/2 hour. see if you get the same symptoms. lets see if it's something you have loading into the main startup. if it carries on, then I to would look at the hardware, either mobo trouble or check the fans to make sure your not over heating.