When was the last time you did a clean reinstall?
Have you cleaned out all the lurking dust on the fans and cooling vanes?
Are the colling vents on the case clean?
How much memory are you throwing at it?
Have you defragged recently?
Is there enough hard disk space left for windows swap file?
If the answer to all these questions are positive, then it could just be that the system is in need of some upgrading.