If you still have a Vista recovery partition then you might want to keep it so you have a path back to Vista, unless you made Vista Recovery disks.
If you decide to keep the Recov partition you can wipe just the install partition using free Partition Wizard bootable CD: Partition Wizard : Use the Bootable CD
Where did you get Windows 7? Did you burn it yourself? You may need to burn another DVD using ImgBurn at 4x speed.
Next remove all but 2gb of RAM to try install.