I've never been happy with Wine (it has been a few years since I've
tried it), but I've been pleased with Sun's VirtualBox. It's open
source (GPL) from the same people who produce Open Office. The URL:

http://www.virtualbox.org/

The down side is that you do need a valid license for XP to use it, as
it emulates the machine and you actually install the guest OS on the
virtual machine.

You can also run copies of Linux inside Windows or Linux inside Linux.