Dell D530 with OS WinXP, Vista upgrade necessary

This post is related to the former question: "DellD530 Win7 upgrade..."

Thanks Philipp, pasxternik and ieee488 for your answers.

Unfortunately I first have the problem to install Vista again (has been installed before company changed it to WinXP) so that it can read the Certificate from the system (stored in BIOS or WinRegistry).

Where can I get a bootable ISO image for Vista (I found nothing in the internet that works)?

Thanks guys