You will have to do a format with a clean install!
To install Windows 7 using the Custom option and formatting the hard disk
To format your hard disk during Windows 7 installation, you'll need to start, or boot, your computer using the Windows 7 installation disc or USB flash drive.
Turn on your computer so that Windows starts normally, insert the Windows 7 installation disc or USB flash drive, and then shut down your computer.
Restart your computer.
Press any key when prompted, and then follow the instructions that appear.
On the Install Windows page, enter your language and other preferences, and then click Next.
If the Install Windows page doesn't appear, and you're not asked to press any key, you might need to change some system settings. To learn how to do this, see Start your computer from a Windows 7 installation disc or USB flash drive.
On the Please read the license terms page, if you accept the license terms, click I accept the license terms, and then click Next.
On the Which type of installation do you want? page, click Custom.
On the Where do you want to install Windows? page, click Drive options (advanced).
Click the partition that you want to change, click the formatting option you want to perform, and then follow the instructions.
When you've finished formatting, click Next.
Follow the instructions to finish installing Windows 7, which include naming your computer and setting up an initial user account.