first of all the programs all have share alot of files (and i mean identicle). Since xp home/pro/vlk share 99% of the same files it is possible.
basically they ran a program from microsoft called cdimage which found the duplicate files and replaced them with crosslinked files (files which appear to logically be 2 different files but share the same physical space on the disk).
It wont work with just any cd's to make an install. hae to meet the previous criteria.