Hi
I see you posted at
http://discussions.apple.com/thread.jspa?messageID=11904845#11904845 too.
Did you run Disk Utility -
Repair Disk +whilst booted from the Install DVD + ? (not repair disk permissions, Repair
Disk, and you'd have had to restart your mac whilst holding down the C key)
did it say there were errors it couldn't fix ?
If so, then you'll either need better disk repair tools, which cost $$ or will need to erase the HD via your Install DVD & then see if you can successfully install again, followed by the appropriate updates.
Back up your data before attempting this, it will remove
everything.
If that doesn't work out, then the HD is faulty, and if you're in warranty, Apple will replace it for you.