windows messenger not loading

I have windows xp sp1. Windows Messenger will load for
the two "limited account" users on my computer -- but not
for me, the computer administrator. It does not appear
in the task bar and when I click on it, the hour glass
pops up briefly then nothing happens.

Interestingly, when I change one of the "limited
accounts" to a computer administator, the same thing
happens -- windows messenger does not load. Anyone have
any suggestions?