Vista users seem to be experiencing problems with Windows Live Messenger 8.1. From what I can gather, it is attributable to two things; DEP and Adobe Flash.
So, what to do next.. Mary Jo Foley of ZDNet has this on her website..
The Microsoft-suggested workaround:
1. Uninstall Flash Player (Click Start->Control Panel->Uninstall a Program, then select Flash Player)
2. Close Windows Live Messenger and any other programs that use Flash
3. Open Adobe public Flash Player beta site (http://labs.adobe.com/downloads/flashplayer9.html)
4. Click “Download Installer for Windows Active X” to install
It’s the Adobe Flash player causing the crashes, concurred Robert McLaws of Windows-Now.com fame. “If you’re on Vista, you have to install the Adobe Flash beta. (Then), it (WLM 8.1) will still crash on exit every once in a while, but nowhere near as bad as it used to.”
The other option is to uninstall Live Messenger 8.1, and revert back to version 8.0, which can be downloaded from here.