SATA Drivers...

When you installed XP the BIOS setting was IDE mode. So XP was installed with IDE drivers. You can't switch afterwards from what I know.

You need to reinstall XP after you set in BIOS the SATA mode for your HDD. Be aware that XP has no drivers for SATA, so you'll have to write them to a floppy and you have to press F6 when the XP setup prompts you to do so.

For SATA drivers check your motherboard disk and manual.

Yes, that's the problem! Floppy drives are almost dead now! Is there any other way to do it?

If you have access to another pc, you can download and install the HP utility to create bootable flash drives. (Sorry, not sure of name or link). You can then copy the floppy files needed onto the flash drive, make sure your bios will boot from a flash drive or usb device and set it to do so first, and it will boot like a floppy and install the drivers needed.

Of course, I believe if you simply set the bios back to IDE mode it will boot just fine. Not sure what SATA native or AHCI mode gives normal users, except it's nice to use an external esata drive without having to reboot with it attached first. A newer BIOS will usually see the drive attached to the sata port automatically and the mode is just for setting the access type - if it was working before, like I said, just switch back to ide mode. If you are reinstalling from scratch anyway, you can start in sata mode.

There is another way.
You can use nLite to slipstream the SATA drivers in the XP installation disk.
All you need is your XP disk, a blank disk, the SATA drivers for your chipset and the nLite application.
Just search on Google for how to make a disk. It's pretty easy.
This way, you don't need a floppy, and no more F6 during install.

However, as tombodude said, you can leave the system as it is right now, with IDE mode in BIOS, as there are no major performance differencies, and go to SATA mode when you reinstall XP again.

What if i copy the drivers on a pendrive and during installation i press F6 and install the drivers from the pendrive? Can that be done? I have nLite for a long time. Never created a disk using that!

Hang on a minute before you re-install your OS. You can switch from IDE mode to AHCI in an existing installation. There are two ways:

#1 Connect your boot drive to an SATA controller that is AHCI independent from the controller you are currently using, switch your original controller to AHCI in the BIOS, boot up and install the AHCI drivers, shutdown, then re-connect to your now AHCI enabled original controller. This requires an SATA add in card or a second SATA controller on the motherboard. Your P5Q Pro has a second controller (Silicon Image Sil5723) that runs SATA ports SATA_E1 (orange) and SATA_E2 (white). If you can keep this controller in IDE mode and boot from it while your Intel ICH10R controller (red ports) are set to AHCI you can use this method.

#2 Find a copy of the Windows XP ICH10R AHCI driver. Either download it or get it from your motherboard CD that came in the package and copy it somewhere easy to find on your hard drive.

Go to device manager, expand IDE ATA controllers and right click on the IDE controller. Select update drivers -> advanced -> don't search I will choose … -> have disk -> and browse to the above mentioned folder with AHCI drivers (select iaahci.inf or similar) . From the list select Intel ICH10R... driver.

After the update windows will suggest to restart your computer. Do so, and when booting go to the bios and enable AHCI mode; save setting and load windows. Windows will now find new hardware and install it.

I used method #2 back when I was on XP on my P5B-Deluxe board and it worked just fine. If all fails, you can go back to IDE mode to boot up again.

The IDE mode in SATA port configuration was designed by mobo and BIOS makers for just exactly this problem - how to install Windows XP or earlier to a SATA drive without a floppy drive. The dilemma is that Windows Install knows how to use IDE drives but not SATA, so it can't install TO a SATA drive. Plus, Install only will accept floppy drives as the input medium to add external drivers. (NOTE: both limits were relieved in Vista and Win 7 - they have built-in drivers for native SATA and AHCI devices in addition to IDE, and they allow USB drives or CD's in optical drives as input devices for driver install e.g. RAID, which is NOT included in the Install systems.) Anyway, the solution built into the BIOS is the IDE (or PATA) Emulation mode of the SATA port. It just makes a real SATA drive appear to Windows as a plain old IDE drive it understands, and there is no problem. Well, a small one: doing this means you cannot use the extra features that true SATA or AHCI modes offer for HDD users. If you don't know what this means, odds are you don't need them anyway. But read up on those extra features and, if you decide they are important, you may have to re-install Win XP to use them on your boot device.