Vista Tutorial - Preventing format

Is there a way to install Vista and uninstalling without having to format the
hard drive? Some people are talking about installing Vista on a partition or
another hard drive. For example, I split my hard drive in two partitions: C:\
and D:\ . Windows XP files are on D:\ that includes: WINDOWS, Program Files,
Documents and Settings, etc... and on the C:\ drive, I put Windows Vista
files. If I format the C:\ drive and then I change the D:\ for C:\ , will I
be able to use Windows XP again? And can I use the D:\ programs when Vista on
C:\ ?
That could be a little bit hard to understand...so for people who don't
understand:

1- Split hard drive into C:\ and D:\
2- a) D:\ drive contains Windows XP files (WINDOWS, Program Files, Documents
and Settings, etc...)
2- b: C:\ Drive contains Windows Vista files
>> Can I use the programs on the D:\ drive when Vista runs on the C:\ drive ? (without Registery Problem)
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3- a) Format C:\ drive (contains Vista files)
3- b) Change the D:\ drive (Windows XP files) to C:\ drive
>> Can I use Windows XP (with Registery Files, etc...) or all is lost ?

If there's a link to a site who could tell me (and everyone who don't want
to do a 300Gb backup) the instructions to install and uninstall Vista without
having to format C:\ .

Thank you