How to Do a Clean Install With a Windows 7 Upgrade

Registry Edit
1. Insert the Windows 7 upgrade disc. Power the computer off and then on again. Choose the option to boot from disc. When the Windows 7 setup appears choose a 'Custom' installation instead of an upgrade. Follow the on-screen instructions to complete the installation. When you enter your product key it will -- most likely -- return an error that Windows could not be activated. If no error is returned and Windows activates, you're done. If you have an error, continue on.
2. Power on the computer where you've installed Windows 7. Open the registry editor -- press the Windows-key and 'R' at the same time and then type: 'regedit' and press 'Enter.'
3. Find the registry key: 'HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/' Double-click 'MediaBootInstall' from the pane on the right. Type '0' in the 'Value Data' box and click 'OK.' Click 'File' then 'Exit.'
4. Click on the 'Start' menu and then type 'cmd' in the search bar. Wait for the 'Command' shortcut to appear and right-click the icon. Choose 'Run as Administrator.' Note, this is only necessary in Windows Vista, in XP you can simply click 'Start,' 'Programs,' 'Accessories' and then 'Command.'
5. Type: 'slmgr /rearm' and then press 'Enter.' Click 'OK' to confirm the command completed successfully. Reboot the computer. Type in the product code when prompted and click 'Activate.' Windows will activate. If it doesn't you may need to download all the available updates and try again.
Double Install
6. Insert the Windows 7 upgrade disc. Power off the computer and power it back on again. Choose the option to boot from the DVD drive. Choose the 'Custom' install feature in Windows Setup. Follow the on-screen instructions to complete the installation of Windows.
7. Enter your Windows product key. Setup will return an error that Windows cannot be activated -- if no error is returned, you're done. Continue booting into Windows.
8. Click 'Start' and then 'Computer.' Double-click the icon for your computer's DVD drive. Choose the 'Upgrade' option from Windows Setup. Follow the on-screen instructions to install Windows a second time. This time when you enter the product key, Windows will detect a previous installation and activate.