If you have a CDROM drive... it's easy to do an "install over".
What might work is to copy the Vista CD to a USB stick and use that to "install over". That's how I got Win 7 RC to install over Win 7 Beta on this machine.
I could send you a Win 7 RC USB stick with the "don't upgrade from earlier versions" disabled, if that's any help - but you'd then have the problem of needing to buy a RTM copy 6 months down the line.
You need to be able to access the "setup.exe" on the USB stick - I don't think you have to be in a GUI, so from DOS might do the job. Not tried it that way, so don't know.
Another thought: if it has a CDROM or a USB slot, a Linux (eg Knoppix) bootable drive may allow you to get in and edit.
Last edited by Saab Dastard; 18th Jul 2009 at 13:30.
Reason: Tidying up