I'd just converted from slack to ubuntu 7.10 last week. The
same thing happened to me, but I'd restarted the install, & it
went fine. I think doing Alt+F4 or Alt+F5 will give you a screen
of what's happening. If you're lucky, you'll see exactly what's
up.

A co-worker had also hit this snag with his video car. Sorry,
I only remember that it was an ATI card.

There's also the possibility your disk is damaged, obviously.