Switching users

I only have 2 users, my wife and me. 2 days ago she tried to switch users to
her and got a black screen with only the cursor. (it was moveable)

I tried to set up another user account and the process started and told me
that it was setting up the account.

The setup got as far as a blue screen, with the cursor, and was stuck there.

I tried to delete users and set up new ones with only the same resuly.
I am sure this is something simple to correct but I need Vista for Dummies.

Can someone please help me?
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Thank you so much, Mike