What would cause a computer to keep freezing up, and how can it be fixed. It also reboots its self a lot. I beleive XP is the OS on it

Could be;

O/S acting up
bad memory
bad motherboard
bad CPU
bad power supply
Heat related issues
Malware

So as you can see, information about the system (hardware/software), exact error messages if any, etc would go a long way towards problem resolution.