If you are able to perfom a clean install of windows 7, the best way is to create a new partition for 7 in vista. If all of your drive is allocated for the Vista installation, you would need to "shrink" the partition, by the amount (size in Megabytes) that you want to use for the new partition. This is done via the Disk Management Console, found in the control panel under; administrative tools/computer management/storage.

Ensure that you are logged on as administrator and before you resize (shrink) the partition, you want to turn off virtual memory/pagefile, then defrag the primary/vista boot drive/partition. This is to help ensure that there are no system files in the area of the partition that you want to free up for the windows 7 partition.

Once you have shrunk the partition, so there is enough space "minimum requirement for windows 7 installation, or greater" you want to go back into the disk managmenet console and create a new partition in the unallocated space.

Here's a link to microsofts instructions for creating a new partition.

http://windows.microsoft.com/en-US/w...disk-partition

Once you have your new partition setup, run the windows 7 installer, select "custom" and select the new partition to install windows 7 on.. Carefull not to select the wrong partition cause if you choose your current boot partition, you could wipe out your current vista install.

Best practice suggests to make a backup of your current vista installation, just incase something goes wrong.

Once the installer is complete, you should have a new bootloader entry for windows 7