This is almost certainly a dumb question, but... say I have Windows 7, with some anti-virus software installed, and I am using Microsoft's Windows XP Mode to run some legacy applications as well. Is there any reason why I would want to install an anti-virus program in XP mode as well?
Not a dumb question at all.
XP Mode is another virtualized environment. If you will be utilizing applications inside it that are accessing the internet, it would be a smart idea to protect it. Your anti-virus on the Windows 7 host can only do so much, you'll need to install separate protection in XP Mode if you want the additional security. I'd recommend doing so.