Before changing any hardware in a PC, you have to un-install those driver before. This is specially important for motherboard and video card.

If you go see in Device Manager under System, you'll find a bunch of driver for all the component of your motherboard. I really think XP is having a conflig in the motherboard driver.

My suggestion is to back up all your data and do a clean installation on your C: drive. If you try safe mode and safe mode don't run, you're usually screw big time.