Shoutbox

I presume you posted this in Messenger Plus! Help & Support by accident, as it's not related to Plus!

Think of anything that could be causing this? Add-ons, patches, firewall, etc.? Remove/turn-off all of them.

Try removing Messenger (with ZapMessenger) or 'Repairing Messenger' if you have Plus! installed, via Add/Remove Programs.

Only other thing I can think of is doing a clean install. Follow these instructions.