Althoug an extremely common culprint is the "MSN Messenger protection" from the anti-virus program AVG.
So, if you have AVG, try to disable its Messenger protection:
Overview > Web Shield > Instant Messaging > MSN Protection
Yeah, that's what I did in both Windows XP and Windows Vista to solve the problem (After a one-hitting-head-on-wall day )
AVG must fix this annoying bug, immediately... -.-
)
