You could always pick up a copy of Windows XP on Craigslist or eBay for cheap. It's a bit of a pain, but you can still do a full reformat install so long as the installer detects the old installation first.
Alternatively, you could buy the copy of Windows XP, skip actually installing it, and work around the fact that you didn't. Legal i's dotted and t's crossed, pain of installing XP avoided.
I tried the WinXP thing but Bootcamp doesn't support XP anymore and it all starts getting difficult without a optical drive (find drive, rip, copy image to usb,...). I ended up just pirating it out of sheer annoyance and since it was the only way I could get Win7 working.