My source was:
http://blogs.zdnet.com/ITFacts/?p=9043
Altavista is such small volume it apparently wasn't worth mentioning. They fall well below other engines like Ask. Under 1% for sure. They use Yahoo! results also so I'd just consider that part of Yahoo!.
Those statistics don't take into account what other small engines power their search by either. Remember that AOL uses Google results, so ranking high on Google also gets those benefits. Yahoo! also syndicates results to other engines like AltaVista, AlltheWeb, and more. Comcast, Firefox, and more use Google. I don't know of a single engine using MSN results.
Again, I'm not trying to knock your results, but high rankings on Google and Yahoo! make MSN search volume almost laughable.