Shoutbox

Very difficult to make sure, only after you ruled out everything:
dodgy virusscanners (that scans everything you type to make sure you don't send/recieve anything dodgy), firewalls, 3rd party programs, addons, plugins, etc... etc... etc..., you can say it is serverside...

But experience told me that it is rarely that and most often is something that you have running along side messenger or bad settings in some program/windows/whatever.