Microsoft did a really awesome thing when they allowed the general public to download a beta copy of Windows 7 before they released it, and let you use it for a relatively long period of time.

Theres only one problem, it was so good in pre-release form that a lot of people, including myself, used it, loved it, installed a ton of software on it, and forgot that it was not offered to be free indefinably. Now they are receiving notices that soon their computer will start shutting down every two hours to get them to purchase a retail copy now that the beta is over.

You might assume that you would be able to just buy a retail copy of Windows 7 retail and pop in the key or upgrade your install in place so you can keep your programs and settings, but Microsoft does not offer this as an option, and since it was beta code, I don't blame them.

Now, I will be the first to say that you should always do a clean install of an operating system if you have option. However, there are times when its just not practical. For example, you only have a short period of time to get someone off the beta and onto a retail copy, and you don't have to all of their software and / or the time to reinstall it back to how they had it.

If you are in this situation, and you want to do an in place upgrade from a pre-release copy of Windows 7 to a retail copy, it is possible. 

I found the directions on how to do this here: http://tech.icrontic.com/articles/upgrade-the-windows-7-rc-to-retail/ 

I tested this on an install moving from Windows 7 release candidate to Windows 7 Home Premium Retail Upgrade.

Remember to back up all of your important data before doing anything that could nuke your OS... ever. This is a process that could nuke your OS. Are we clear? But since your not a total n00b you have two copies of all your essential files, so this shouldn't even need to be stated.

Now, here is how you do it. (I am going to assume you are using (Windows 7 Home Premium)

Step 1. Put your Windows 7 Home Premium retail DVD

Step 2. Create a new folder on your hard drive to copy the contents of your Windows 7 disk to. Lets go with creating a new folder on your c:\ drive named "retaildvd"

Step 3. Now go to your new folder, which is c:\retaildvd, leave it open

Step 4. Open up your My Computer folder, right click on the Windows 7 DVD, hit open

Step 5. Copy all of the contents of your Windows 7 DVD to your new c:\retaildvd folder

Step 6. Get yourself a beer and wait for it to copy

Step 7. After the copy process is done, go to the c:\retaildvd\sources\ folder and find a file called "cversion.ini"

Step 8. Right click on this file, and hit edit

Step 9. Change the value of "MinClient=7233.0" to "MinClient=7077.0" (this tells your retail installer that its cool to upgrade your existing copy of Windows in place)

Step 10. Assuming you didn't buy Windows 7 Ultimate, and lets be honest you didn't, we need modify your registry. Go to your start menu, type in the search area "regedit" and hit enter.

Step 11. Hitting the arrows next to the correct folders and navigate down to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version, and click on that "Current Version" folder, not one of its subfolders

Step 12. Double click on the "EditionID" icon and change it's field to say "HomePremium", then hit ok

Step 13. Double click on the "ProductName" icon and change its field to say "Windows 7 Home Premium", then hit ok

Step 14. Close out of the registry, you don't have to save anything, its just magic like that

Step 15. Go to your c:\retaildvd\ folder and run Windows setup

Step 16. Go all the way through this Windows 7 setup doing it as an upgrade. After a while your computer will restart, then run some more, then restart, then run some more, then restart, and run some more. An in place upgrade takes longer than a clean install.

Step 17. Get more beer, and wait

Step 18. Follow the rest of the prompts, give it your new Windows 7 Home Premium (or whatever version you got) retail CD Key, and you are pretty much done.

Microsoft chose not to officially support this process of in place upgrading from the release candidate, so I can't promise it will work for everyone in all circumstances. It did however, work for me, and its worth a shot if your only other option is a full blown clean install. 

I am writing this from memory, so if I made a mistake, please let me know in the comments and I will fix it.

Enjoy.