I strongly recommend XP if you want Windows. An upgrade version of XP is under $100 dollars, and you can get OEM versions of XP Home on Ebay for under $70, compeletly legal and legit.
I'd be careful buying XP from odd sources. I recently bought a copy of XP Pro for $99. It's a real Windows CD, with the hologram and all, but it is intended for use in a corporate licensed environment. You cannot use Windows Update with it. The "Advantage" applet says it's counterfeit.