Is your computer a brand name computer (Dell, HP, etc)? If yes, create recovery discs before you do anything else. This way you can always revert to original factory state, There are instructions in the documentation that came with your computer.
BACKUP YOUR DATA!!! Once hard drive is reformat you will lose all your data.
You must own a copy of Windows XP with valid license key.
Give more details about games and apps you are running. There might be updates for win7.
Few things you can try:
1 - Run your games and apps with XP compatibility mode. Right click on application to create shortcut. Right click on shortcut, select Properties. Select Compatibility tab, and check the XP mode checkbox.
2 - Since you have Win7 Pro, it has a virtual XP mode, Basically creates an window that is running XP pro in it. In this virtual OS you can install you games and apps. Below is the link from microsoft to download XP mode.
http://www.microsoft.com/windows/virtual-pc/download.aspx
3. If you have a desktop, you can install a new hard drive and put XP in it. Be sure to have the computer off when installing hard drive. And have the first hard drive unplugged when installing XP on the second drive. Once installation is done, turn the computer off and plug the first drive back. Turn the computer on and you need to hit F8 (or some other F-key) to manually select which drive you want to boot from. You can also leave the original drive unplugged if you really don't want to use Windows 7.
I would not recommend replacing completely Windows 7 with XP home. You might have issues with some hardware if there is no drivers for windows xp.
Good luck.
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