the last time I had any problem of this sort it was related to security audits, i believe there are several conditions involving security audits that will cause windows to prevent you from logging on.
It doesn't sound like this is your exact problem but it might be something worth looking into.
can you log on using the XP default Administrator account?
Intel core I7 920
6GB OCZ platinum 1600
Samsung 1TB F1 Spinpoint
Samsung SM2443BW 24"