Do not buy Windows 7 on eBay. We see too many cases here of consumers who were ripped off. I would cancel it if you still can.
Have you considered becoming a beta tester for new Windows 8? You can download and install it for free, participate in our EightForums where you can become an expert and be offered some great pre-release deals like the free Signature Edition we were awarded for holding Windows 7 release parties, half price Upgrade versions just before release, etc. It's fun, a veritable gravy train, and perfect for the cash strapped.
Other than that Ubuntu has many free flavors of OS if you like Sanskrit.