Not really OT so may as well stay public, (I'm sure SJ or f0dder will hit me if I screw up
Before you do anything, I would really recommend you do a HDD image or at least an image of the XP partition/MBR and make sure you have a bootable recovery disk for your imaging software AND
that it works.
First off, specifically, which ASUS P5Q motherboard, ie. the 'P5Q', P5QL-EM, P5Q Deluxe, etc, and what SATA port is the system drive plugged into, eg. the P5Q has Intel hosted SATA ports (6) and Silicon Image hosted SATA ports (2).
Once you know those two things then what I would do is:
1) BIOS settings for the particular ports should already be AHCI but it doesn't hurt to check.
2) Download the 7 x64 AHCI driver for the Intel SATA ports from ASUS and copy the contents of IMSM_V8901023\Driver\Disk to a flash drive.
This part's from memory:
3) Leave the flash drive plugged in and boot the Win7 DVD, IIRC, when Setup sees it needs more drivers it will automatically scan attached media: flash drives, CD/DVD, floppy, etc. Or if it's connected to the internet you'll be given the option to download any updated installation files, use that if you're not sure.
4) Select Custom installation so you can choose the partition to install on.
5) That should be it.
Windows will recognise the earlier installation of XP and you'll have an option in the boot menu to select it, Older Windows System or something like that.
If you find that the installation is taking more than an hour, (CPU/RAM/HDD speeds affect it but not too much), then it's likely that the AHCI driver wasn't loaded.
The Intel P45 chipset should be pretty well supported by drivers already on the DVD, it's been around a while.
If you want to check beforehand you could download the Windows 7 Upgrade Advisor
and run it under XP, it should highlight any problems with devices - it checks for both x86 and x64 compatibility.
EDIT: Dear God my grammar's getting bad....must be old age.