My friend also got the same problem running yahoo messenger in vista, although other friend of mine use Dell XPS and run well with msn messenger.

My suggestion is to download patches for vista :

Search it at google ; keyword vista patch

Or you can go to microsoft website

to get the msn that I think is compatible for vista.

Other harsh think you can do is to format your harddisk, then reinstall you vista. Although you already do a registry cleanup, there might be a slight of xp stuff left. Just one of them can crash your system, careful with that.

last question : When did you buy your vista ? Coz if you buy it when it was just being released, you should look for patches because the just released program usually have a lot of glitches.