1. If you have Vista on drive C and Windows 7 on drive D, download VistaBootPro from click here and install it in Vista.
Go to Manage OS Entries > Highlight Windows 7 > Click X > Confirm the deletion. The partition that occupied Windows 7 can now be formatted via Disk Management.
2. If you have XP and Windows 7, you will have to recreate boot.ini, have a look click here