Removing Vista and installing XP under unusual circumstances

[Popinjay]

I didn't know whether to post this as a vita question or an xp question since it involves both.  I put it in both places in hope for an answer.

I have vista installed on my C: drive, and on my D: drive I have XP installed (both are part of the same physical hard drive).

Here is what I want to to:

Remove Vista from the C: drive and install a later version of XP onto the C: drive to replace it.  It is important that I do this without destroying my ability to boot to the D: drive or ruin my information on the D: drive.  Remember that they are all the same physical drive.

I tried to find an answer to this online but all I could find were instructions on how to do it if vista was NOT installed in your C:drive.  Unfortunately mine is.

What can I do?

Any help would be appreciated.

Thanks,

Popinjay

[Geek-9pm]

This topic has been beat to death.
So please look over this before we start all over again.
I am the one that says it can not be done,
Everybody else days it can.
Read and decide for yourself.   
http://www.computerhope.com/forum/index.php/topic,68488.msg446770.html#msg446770

[patio]

In future there's no need to Double Post for assistance...