Sounds like you are out of luck. If you still had the OS installed you could get it out of the registry.
You can get
Vista Home premium for $99
Vista Ultimate OEM for $179 (which I would only recommend over home premium for it's simple built in back up, which you could do cheaper with third party software)
XP Pro OEM for $139
Newegg.com - Operating Systems, Microsoft Windows Vista, Windows Vista 32, Microsoft Windows XP, Microsoft Windows Media Center, Linux

When you get the new OS installed, find a back up solution that works for you. It's not if you'll lose data, but when.