Is there a reason why you specifically need windows 7, or are you just looking to upgrade your operating system and run windows software?
I'm just thinking that if you just want to run Windows software inside a modern operating system, the obvious free option is Linux. It's a change from Windows, but it is free and a lot of Windows software runs on Linux emulators like Wine. You can run Linux from a CD to see what it's like before you install it, and it can be installed onto a separate hard disk partition so you can boot either windows or linux. So that would be my suggestion.
If you need Windows 7 specifically, I guess the only option is to import a copy. But that's not going to be free. Unless you have a friend with a spare copy the only free ways to get Windows tend to be dodgy illegal ones
Wenchy