Windows Messenger has encounter a problem

Hello,

We are using LCS 2003 and WM 5.0 in our company.
One user is encounting a problem since recently.
When he starts Windows Messenger, there is the error window:
"Windows Messenger has encountered a problem and need to close. We are sorry
for the inconvenience.
If you were in the middle of something, the information you were working on
might be lost."

Everything was working fine before and this error occurs sometimes at
Windows Messenger start up but we don't why and it is not systematically.

We try to install Windows Messenger 5.1 but it doesn't correct the problem.

The computer OS is Windows XP SP1.

Thank you.