How come my laptop shuts off after about 2 hours of usage?

I think that it is an overheating problem considering my laptop (ONLY recently) seems to be getting very hot, to the point it needs to shut down. But this is the strange thing: I had Windows Vista installed on this laptop (Acer Aspire 5100) earlier, and my battery almost never heated up to the point it needed to shut down, or even heat up to an extreme level.... But now that I have downgraded to Windows XP SP2, it seems to shut down MUCH more often. But I did find one thing of concern. Around the same time my laptop started shutting down, I installed an application called 'Game Speed Changer' and that whenever I had used it, it slowed my computer down immensely, and then when I went to close it, the BSOD would appear. Every time. Any insight to my problem is greatly appreciated... I would love to have this fixed before I go back to school! Thank you. (Notes taken on Laptop)