If youre running your servers on XP pro you have bigger problems.. And I highly doubt MS will start letting you rung three instances of their server product for free.
What's the problem? I use it basically as a file repository and central point from which to create offsite backups, as well as a host for the SQL database our contact manager uses. I have a small company with six computers on our network, not Google's IT department. XP has worked fine for those purposes.
When I upgrade the hardware next, I might like the ability to segregate the contact manager function from the data storage/backup functions, as well as segregate those from Internet access and have the ability to take my "server" home with me if I want to use it there. It would be a bonus if I could do so without extra XP licenses, but we're 100% software legal here so if I need to buy the keys I'll do it. I was just pointing out that maybe Virtual PC would allow one XP license to be used in such a manner, in which case I'd almost certainly adopt it.