Did you download SP3 through a bit torent? It is likely that the copy you got is corrupt or you may have hit a bug in the code. I personally avoided SP3 on the web, since as soon as it was leaked out in July, MS shut of the leak, and there may be reasons for why they are not ready to release it,and you hit one of those reasons.
Looks like you are going to be looking at a rebuild...
You could try an emergency repair floppy from another system of XP Pro, but this could cause even more problems than when you started.
If you have important data on the hard drive, I would install a new or used HD and install the OS to it, then reintoduce the original hard drive as a slave to get your data.