Perhaps X is crashing right after you enter your credentials? Did you check the log files? Popping over to a virtual terminal and checking /var/log/Xorg.0.log and /var/log/messages might give you a good start to see if something's going on.
Also, just curious...have you tried SuSE 10.0? If you have access to it, it might support your hardware a bit better and have less bugs. Just a suggestion though...
As for restoring Ntldr, you can boot up off an XP disk, go to a recovery console, and run fixboot or fixmbr. That'll restore your ntldr, but it'll also overwrite grub...you might want to read this forum
. It's got some good info in it.