You can't buy Windows7 from Microsoft anymore but you can buy it on ebay for under $100. Windows7 is still supported with regular updates from Microsoft and is very similar to XP from the user's perspective. I've used Windows7-64 bit, Professional for several years and am very happy with it.

If you don't have a recently made pc, you may not be able to use Windows8 anyway. Most reviewers recommend staying with Windows7 if you already have it and not upgrading. There are issues.