What do I do to get rid of this message?
Don't turn the computer on, of course :-)
Is there no way at all you can find a copy of XP?
If not, what about Ubuntu? Do you have a copy, or can you download a copy?
There is the ability to re-build an MBR from Ubuntu.
There are other solutions (such as bootable tools like GParted), but I don't recommend you try them until another user can verify they're the right tool to use. I fixed my MBR using GParted when things went a little pear-shaped and everything turned out fine. That said however, GParted can destroy data so consider Ubuntu or wait for someone even smarter than I to come along