Switch user black screen

Have been using XP for years. Now find that when switch user is used, just
get a black screen with the mouse. Have to reboot. Can not have 2 users
logged on together. Switch user is enabled. Doesn't matter if it is an admin
user or not, or what program(s) are running, the first user is the only one
that can be used unless they log off. Any ideas.
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flash