If Vista came pre-installed and they didn't give you a disk, then most likely
there's a restore partition on the hard drive. That may or may not have a
"system disk" type folder in it.
If you're just looking for a way to guard against some sort of data loss in
the event that your HD crashes or something, I'd recommend creating a clone
of your HD using Norton Ghost, Acronis TrueImage, or equivalent software.
That way if you have some kind of drive error, you could get a replacement
drive and ghost that image onto it.
Regards,
Jim