Get a rescue CD (e.g. from here: http://rescuecd.sourceforge.net/download.html)
, boot your computer from it, then login as root (if you use the rescue CD mentioned before, the password is rescue
, then mount your hard disk, chroot to it and set a new password for root:
mount -t auto /dev/hda1 /mnt
Reboot your PC (don't forget to take out the RescueCD), and should be able to login as root.