Unless you have a full version of XP, 2000 or Vista to reinstall, then you will have to purchase the full version.
Everyone has had plenty of time to use the Beta & RC version and save for the retail release.
Are you running Windows 7 RTM downloaded directly from Microsoft? If you are not, you may experience issues outside of Microsoft's support.
Currently, Windows 7 RTM is available to all who have a TechNet Plus and MSDN Plus subscription. If you have a TechNet Plus or MSDN Plus subscription you have access to download Windows 7 RTM and receive 10 product activation keys. To learn more about how to obtain a TechNet plus subscription, please go here: http://tinyurl.com/nnm26v
Microsoft Windows Client Support