Windows 7 - Accessing old Vista install on external harddrive - UAC

"You need to allow yourself full access to the hard disk.
The following procedure will only work if you're logged in as an administrator.

The first step is to take ownership of the files.
Start a Command Prompt box (cmd) as administrator, and enter:

takeown /f /r X:\

Then give yourself full rights on the file:

cacls X:\*.* /T /G username:F

For more info on cacls see:
Security from the command line with CACLS.

If you're having problems using cacls, a more evolved Visual Basic script exists:
Xcacls.vbs to modify NTFS permissions."

Have you tried setting up an account on your new Windows 7 install with the same username and password as the old Vista machine?