Hey! So, I'm going to get Windows 7 for Christmas, but I already have Windows XP on my new rig. If I install Steam on XP, do a clean install of Windows 7 (not an upgrade), can I still play those games from Steam on Windows 7?
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It makes sense that you could. I'm building a new rig and using my old HDD with Windows XP on it for a while then buying a new Hard Drive and Windows 7 when the prices go down. It should work as I have wiped my hard drive before and Steam keeps your profile and all you games. You will have to Install all of your games again though
So basically what you are asking is if you had two hard drives and Windows XP is installed on the C: Drive, then games are installed on the D: Drive, you do a clean install of Win 7 on the C: Drive, will the games still work?
The answer is no because programs / games make changes in the Windows Registry. Without those Registry changes (called keys), the operating system will basically not recognized those programs / games and you will not be able to run them.
The only exception that I know of may have been either Morrowind or X2: The Return.
The games will work, but Steam will have to go through the first time installation process again with each game. You shouldn't need to redownload anything, just do a clean install of Windows 7, install steam to the path of your steam folder on the secondary drive, and login, all your game will be there, they just need to go through the first time install again when you launch them in Windows 7.