There are plenty of simulators for various kinds of OSs and browsers. I just searched for "Win XP emulator online" and found this: http://virtualdesktop.org/complete/xpsp2/index.html. The question is whether it is accurate. You would need some research to form your list of reliable emulators.
Besides, public cloud vendors (Amazon, first of all) usually allow creating free virtual machines in their clouds for a trial period. You should check what options you can find there. Of course, they will not provide you a wide range of versions, but still worth checking.