If you've gotten to a login screen, grub is no longer involved. It would seem you have issues with your users, and video, which would indicate some sort of partition corruption, or possibly memory problems. I would start by running an fsck on the partition, booted from the liveCD, logged in as root. From a terminal, with the partition unmounted;
[root@localhost ~]# fsck -f /dev/hdb1 <Enter>
When the process completes, try to boot to the installed system again. If you can then log in as root, try;
[root@localhost ~]# video <Enter>
If that works you should be able to set up your video card again to get back to the GUI desktop. If you still can't log in, try running memtest86 from the liveCD start menu, for about an hour.
Post your results.