Are you able to create a new admin user account?
Although I'm not running Windows 7, I had a similar problem with XP a few months ago, and it turned out to be a virus which stopped me accessing all my hard drives. There is a free program out there which disables autorun on all hard drives (at least that's what it said), thus stopping the virus from activating itself. I can't remember what it's called though (wish I could), but some other users might know what I'm talking about.
Worked for me so give it a go. Hope this helps.