Shoutbox

I've had the same problem (Windows Messenger).
I used Messenger Disable to kill it, but I found that disabling it 'For the current user only' doesn't work, and disabling it 'System-wide' does work, but causes MSN Messenger to hang for a few seconds (about half a minute) when it starts. Very annoying.
Try switching between those two though; it might work for you, since I use alot of patching and tweaking.. it might be because of my odd config.

In the end I had to just uninstall it..
Hope you find a better way, I searched the entire hard disk and even did a search through all the files of my hard disk for the entry msmsgr.exe (or whatever it's called - forgot the exact filename). No results.. Must be hardcoded in Windows or something ? I wondered about how it started and if there is no apparant way of disabling it.. wether it wouldn't form a huge security risk.. suppose another program could use whatever startup method this one uses.. say.. a Trojan..
Meh @ Microsoft.

* lhunath gives an evil look in microsoft's general direction.