if there is another OS such as win2k or linux, then we have a solution
startup another os
cd into winxp dir--windows
cd system32
cd config
ren sam sam.bak
or linux
mount /dev/hda /mnt/tmp--hda or sth depends on your winxp partition
cd /mnt/tmp
cd windows
cd system32
cd config
mv sam sam.bak
reboot your computer