Even with a valid key, you may have to activate it via telephone, as the online activation servers for Windows 7 are occasionally unreliable.

You can sometimes find unopened "new old stock" (DVD boxes) on eBay. Ensure you are getting the Retail version rather than an OEM version, as OEM keys are tied to the first motherboard they are installed on. Is there a Student Discount?

Microsoft and major retailers (like Amazon or Best Buy) no longer sell Windows 7 licenses directly.