Sorry I have to keep this message ultra short as I am already late for a meeting.
1. You need to find out and download the SATA drivers for your HDD, these will be required at the start of installing XP (F6 etc)
2. You may very well need the Vista disc to carry out a "very small" repair after installing XP, which is quite a common occurrence and only takes 2 minutes.
Sorry I have to go now, but will be back later.
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edit
Let us know when you have the SATA drivers.
Actually I should of said "SATA Controller" not driver.
Although I'm not sure how you find which one it is that you will need, I think it will have something to do with the make and possible size of the HDD, so if you post those details hopefully someone can help you with that part.
Although on some newer mainboards apparently this is not necessary.
However, as you are saying you want to install XP on the second drive (providing this is not a partition) actually makes it a lot easier and you "may not" need the vista disc.
Initially you could try just installing XP and see if it recognises your HDD, if it does then you will not need the SATA controller and so skip step 1.
Step 1:
So all you need to do once you have the SATA controller loaded onto either a floppy disc or CD is to go through the motions of installing XP, but keep a careful eye on the bottom of the screen at the start of the installation, because you will be asked if you need to install addition software (or a message similar to that) and to press F6 to accept, later on it will ask for the destination of the addition software (this being the SATA controller)
Step 2:
To install XP place the XP disc into the drive and boot-up the PC, before you get to the setup stage you will be asked which drive to install XP onto, so chose your second drive.
Once the install has completed you will probably then have to change the BIOS setting to boot from the Vista drive, then you can install EasyBCD, "this small program allows you to select which operating system to boot into each time the PC is started" it also allows you very easily to change, not only this setting, but how long the notice will appear on your PC screen asking which operating system to boot into.
The process, as said earlier is easier because you will be installing onto your second drive (not a partition of the main drive)
So basically, that's all there is to it, but I would strongly advise that you backup your main HDD by creating a recovery image before starting and also if possible get hold of a Vista disc.