I don't use MSN, but the same thing happens to me in general. I don't usually notice until I'm typing an email or a document, and I don't know how to fix it. I usually restart my computer as well and the problem goes away until the next time, just like you. But for me, it's not just the ? symbol...it's also the quotations and some of the symbols above the numbers. The only thing I've found is that it seems to happen if I've left my computer on for too long. Sorry that I can't help, but you're not alone in it happening.
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