Logged on at another computer error

I am running Windows Messenger and I keep getting the
message that I've logged in at another computer and then
it kicks me off. Seems like it happens every 15 minutes
or so, but then sometimes it won't do it for a few hours
at a time.

I was afraid that someone had gotten my .NET password, so
I changed it but the problem continues. I do have
Windows Messenger installed on another computer, but even
when it's shutdown, it still does it.

Any ideas?