hmmm...i had faced a similar problem once, but i wasnt even able to boot into the safe mode...but u can try this:
if u have the bootable win xp cd, boot into the safe mode, put in the cd, run>cmd and enter sfc /scannow
if that doesnt work, boot with the windows bootable cd and perform an installation over the original installation (not a new installation)
but whatever u do, ur win xp will never be the same....it will be slowed to a great extent...(if u had any sevice packs installed, things will get worse)...the best option is to backup ur files on a different partition or burn them on a cd and perform a clean win xp installation.