Today's the last day to pre-order Windows 7 Upgrade discs for half off. But Microsoft almost always offers upgrade discs for existing Windows users for a lower cost than the full install discs. So even if you don't get your order in today, you can probably save a few bucks by ordering Windows 7 Upgrade instead of the full version. The question is: will you be able to install it?

Microsoft has already stated that Windows Vista and Windows XP users would be able to use the upgrade discs. But this week the company also confirmed that those of you who have been using Windows 7 RC will also be able to upgrade to Windows 7 final using the upgrade discs.

There is a catch. While the Windows 7 installer won't overwrite your files, it will copy all of your existing data to a folder labeled WINDOWS.OLD, which means that you won't be able to run most applications without reinstalling them. But at least you won't lose your documents in the upgrade process.

Now here's the bigger catch: While you won't need to downgrade from Windows 7 RC to Vista or XP in order to install Windows 7 final the first, time, you will need to install one of those operating systems and activate it first if you want to do a clean install in the future. Because the Windows 7 upgrade disc will only work with an activated version of Windows. That means you'll need an activated version of Windows XP, Vista, or Windows 7 RC to start with or the disc will be useless.