Reverting to a Previous Version...
Given that some people may be looking to remove Windows 7 from their systems I would like to offer a few notes:
1.) Microsoft does not support downgrading Windows, so if you wish to revert to Windows XP/Vista you must format the drive, which means you will lose all personal files and settings. Please back everything up before reverting.
2.) Most manufacturers are no longer shipping recovery system CDs/DVDs with new computers, so if Windows 7 was preinstalled I would strongly recommend burning the CDs/DVDs yourself from the hidden partition the manufacturer created on your hard drive. To do this you should find an option in the Start Menu along the lines of burning system restore discs. Without these discs you will be unable to reinstall Windows 7 at a later date, if you so choose.
3.) Many people have reported being unable to install a previous version of Windows on the same drive/partition as the current installation. If you find yourself unable to reformat or delete the partition using the Windows CD/DVD, you can use the free utility DBAN to wipe your hard drive and give you a clean start. If creating a bootable floppy or flash drive, please note that you need to run the executable you download in order to create the bootable diskyou cannot simply copy the .EXE onto the disk. On the other hand, if you are creating a bootable CD/DVD please note that you must burn the ISO as an image file, not just as a data file.
4.) If Windows 7 was preinstalled, understand that the computer manufacturer most likely will not support your installation of XP/Vista, meaning they will neither provide full technical support nor provide all of the required drivers. Make sure such drivers are available from the third-party manufacturers (ATI, Nvidia, Realtek, C-Media, Lucent, etc) before reverting to XP/Vista.
5.) Please note that if you have an OEM copy of Windows XP/Vista from another computer you may not install it on your new computer. OEM and otherwise preinstalled copies of Windows are tied to the specific computer and cannot be legally moved. You can only transfer the license to the new computer if it was a full retail copy.
6.) You may also consider dual-booting Windows 7 with another operating system, such as Windows XP or Windows Vista, letting you switch back and forth between the two at will. Have a look at the other tips in this thread to get started.
If you have any additional questions concerning reverting to a previous version of Windows feel free to ask.