Boot with Windows 7 dvd, select custom install, disk tools, format the drive and continue the installation.
Don't forget to backup any data you need first, use the Windows Easy Transfer Wizard to save and later restore.
Your best bet is a clean new installation to avoid any problems. Regrettably, you will have to re-install most of your programs.
Backup all of your data to an external hard drive or flash drive.
Install clean installation of Windows 7 and then keep the Windows 7 Product key. You can activate it via Phone Activation system and at the same time, the copy on the so called "old computer" *your old windows system is deactivated.
Phone Activation system
1. Click the Start button, type "SLUI.EXE 4" (without the quotation marks) in the search box, and press “Enter”.
2. Select the neatest location, and click "Next".
3. Please click "Continue" when the User Account Control dialog-box appears.
4. The Windows Activation will generate a toll-free phone number. And the installation ID will be displayed in the following format:
xxxxxx – xxxxxx – xxxxxx – xxxxxx – xxxxxx – xxxxxx – xxxxxx – xxxxxx – xxxxxx
5. Call the number to activate Windows 7.