I am having exactly the same problem, my XP is not pirated but as it came with my PC when I bought it.
I was trying to remove the about:blank hijacker from my system, deleted several things flagged up by Hijack this & Ad Aware 6 and restarted the computer, when I did this I could not get past the welcome screen.
Does anyone know how I can fix this as I really need to computer for work.
Thnaks