I think I found the solution of why yahoo messenger vista preview was having the problem “yahoo.messenger.YmApp has stop working”. this was because a feature of “windows Features” was disable and it cost yahoo messenger to stop working. please let me take you step by step on how to get there.

1. go to control panel
2. go to programs
3. go to Turn Windows features on or off
from there you should see a window with features/folders with check/uncheck boxes

4. GroupPolicy Preferences CSE (if showed)
Windows DFS Replication service and
XPS viewer - found under Microsoft .NET Framework 3.0

hope it helps the problem