If you can boot into safe mode without the computer locking up, 9/10 times it will be a software/windows problem, and NOT hardware.
My suggestion would be to put your windows xp cd in, reboot and let it start windows setup. If it doesn't boot from cd rom, go into your bios, make sure CD ROM is before your hard drive in the boot device priority list. Save and reboot. Once in windows setup itll have 'repair windows using the recovery console', down the bottom will be the keys you can press for the different options. You want to CONTINUE at this screen, dont repair just yet. Then itll begin to load a few things, and itll come to a screen with a disclaimer on it, Press f8, then the next screen appears, press r for repair (doing it off the top of my head might not be in correct order)
Then let windows do its thing and you'll be away laughing. This repairing of windows wont format your harddrive, itll appear just as it was before you repaired, however some programs will need to be reinstalled.