Can't access any other user in XP?

Now, I can properly log off and
get the three users names on the welcome screen but when I try to click
one on (eg, 'guest account') I just get a quick blue screen and back to
the welcome screen.
For two weeks, I have tried to solve another problem that would not let
me switch users, so tonight I did a system restore and got this. Why?
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Karen Shelton2010-12-12 23:17:57