How are you trying to reformat? Exactly how are you doing it, using the original XP installation (recovery) disks or some other method?
You are receiving a stop error, in your case error number 0x00000050, or 0x50 for short. Information regarding the possible causes, how to troubleshoot, and how to recover from this error can be found HERE
. Scroll down the page until you see 0x00000050.
Just a warning: this may not be easy to diagnose. You will need to read VERY carefully, and follow instrucitions very closely. Please ask questions here if you get stuck!