Ti's a strange one. I'm doing a bit of sideways thinking on this one, the only possible connection I can think of is Internet Explorer. I know the MS office packages require IE to be install for it to work, I'm wondering if yahoo messenger also require IE for it to work as well, which may cause a conflict. So it might worth trying a repair of IE, to do that go to Control Panel then Add/Remove programs and click on Remove IE and when the three options appear, select repair.
Even if I'm wrong, it won't hurt to try.