Well it could be a number of things. Usually best to get an idea of when it freezes. Such as; computer idle, computer running regular not demanding software like word, excel, etc, computer running games=most power required+most video resources required.
I am guessing you installed the most current video card driver for XP, you can get it from nvidia.com for free.
Power supply could also cause problem, like when running games. More power required for intesive graphics. you can try to use this calulator for a quick idea if your power supply has enough wattage. http://www.thermaltake.outervision.com/
Your current power supply should have a sticker on it showing available output in watts or voltage and current.