I have seen this issue occur on friends and family member's computers too. One kept receiving 'Windows Live Messenger has stopped working' crashing on booting Windows Messenger 8.1. Some of the earlier revisions to Windows Live Messenger were not fully Vista Compatibile.
My recommendation to all those with crashes is uninstall 8.1, restart, install 8.0.
Works almost the same as 8.1 and most importantly is fully Vista compatible. It also basically has all the same features. Voice chat, text chat, emoicons, web camera chat, file transfers, picture-drawings, etc. In fact, I am not really sure what changed in 8.1 over 8.0
except for maybe the 'Shared Folder' feature possibly. Never really gotten into having a shared folder with friends yet though.