Need Help with Strange Messenger Problem

I've experienced a strange problem with newer versions of Messenger.
When I install a new version, everything seems OK. I can see all my contacts
and they can see me. When I post a message my contacts dosn't receive
anything, and the same the other way around. I thougt they ignored me to
start with, but soon I found out that nothing went away, although I think
it's sent successfully.
I receive NO error messages, no strange thing at all.

If I remove the last version and use the XP integrated version, everything
works fine agn. Does anyone have a clue about this malfunction.
I also tried to disable Symantec Firewall, but nothing changes. Please help me

Björn