Brand new Windows 7 computer - Can't open explorer.exe, control panel, or help. Can't see or save to C: or D: drives. Got the computer last night, and it worked fine. But tonight I can only access IE, Media Player, etc.

Here's some bacground info...

I only have the default user account at this point, so I have admin rights.  I haven't really done much yet since I just got the computer last night.  I was going to transfer files from my old Vista computer tonight, but I can't even open up explorer on my new Windows 7.  I tried rebooting, but it didn't help.  I also ran the SFC; it did not find any integrity violations. 

When I click on the folder icon on the task bar, I get a dialog box with a heading of "explorer.exe" and a message stating "Windows cannot access the specified device, path, or file.  You may not have the appropriate permissions to access the item."  If I try to print a web page to XPS, it doesn't commit the save on the C: drive (OS and Program files) or on the D: drive (data).

I have no idea why it is doing this!  It is a brand new system.  Last night I installed Windows updates and changed the desktop background.  I haven't installed any programs or transfered data.  And it was just fine last night; this problem started when I turned it on tonight.  I would greatly appreciate any help!