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.
Having the same problem myself, i have tried fresh install etc totally confused as it worked fine a week ago.
Suggestions i have had are that its hard drive related but all scans on it seem o.k.
Are you having problems with updates and downloading progs?
Nope
Ah right, didnt even think about it possiably being a virus
Can you remember what program you used?