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#21
I don't believe there is any reason why you cannot run Win7 on a letter K drive as any programs already refer to that letter.
So if you don't want to reinstall you can boot the Win7 DVD repair console or Repair CD, open a Command line by pressing Shift F10, then use Diskpart to mark Win7 partition "Active" and then mark Vista partition "Inactive": Partition - Mark as Active
Now power down to unplug Vista HD, and either swap its cable to the Win7 HD or set Win7 HD as first HD to boot in BIOS setup, after DVD drive.
Boot back into Win7 DVD Repair console or Repair CD to run Startup Repair up to 3 separate times with reboots to write the System MBR to Win7 parttion since it is marked Active: Startup Repair - Run 3 Separate Times
When Win7 starts up, you can again power down to plug back in Vista HD to format partition in Disk Mgmt. It should not interfere with the boot any longer if you marked it Inactive after you marked Win7 partition Active. If Vista interferes, use DISKPART from DVD/Repair CD to "Delete Partition Override" the Vista partition.