This is more or less a reminder for me, most of you will probably already know how to do it...
apt-get install qemu
Create a (resizable) image which will hold Windows XP. The installer chokes if the image is smaller than 1.2 GB or so, but that's not too much of a problem; the "qcow" image format will only take up as much space as is really needed, so the image will be very small in the beginning (not 1.2 GB big!).
qemu-img create -f qcow /path/to/xp.cow 1300M
Insert the install CD, and install Windows in the QEMU image:
qemu -hda /path/to/xp.cow -boot d -cdrom /dev/cdrom -m 384 -localtime
After the install has finished, shut down the QEMU/Windows; from now on you can boot it (without having to insert the CD anymore) with:
qemu -hda /path/to/xp.cow -boot c -m 384 -localtime -k de -usb
Type "qemu -h" for more options.