Install home, download, burn cd, do clean install of Win 7.
If you can download win7 on another computer you can do a clean install of it without a previous os.
Install win7 WITHOUT putting the product key in. let it boot into windows but dont activate or do anything else. Reboot to the install cd. Win 7 will see the previous install of Win 7 as a qualifying upgrade route, whereby you can reinstall win 7, enter product key this time and activate.
Works, did it with utlimate 7 after a botched clean install.