First off, I'd back up anything important just to make sure you don't lose anything. Then, boot the Windows 7 CD and go to the advanced repair options, and then choose the command prompt.
From there, you can rename the Vista folders to something else temporarily:
"ren Windows Windows.vista" etc...
And then you can rename the directories ending in ".old" back to their origional state:
"ren Windows.old Windows", "ren Program Files.old Program Files" etc...
Once this is done, you'll need to repair the boot information - the easiest way to do this is to exit the command prompt and follow the Windows 7 startup repair tool