My wife had this problem on her Vista Home Basic system. Based on informantion I found from other forums I used the Vista Restore (hold down F8 at startup) and selected a restore point before she started having the problem. That process worked fine and no user data was lost.
Note that the Vista Repair option did not find any problems.
Every other user post that I saw said the Restore fixed the problem for them too, so I'm adding my concurrence for how to recover.
