Alternatives for G4-Users:
Maybe you can use Q which is based on QEMU but it will be slow (XP might even be useable but Vista (I tested beta2(?) on a 1.6 GHz single G5 and it was not usable)
http://www.kju-app.org/
Maybe bochs (the open IA-32 Emulator) can help too but I didn't look into...