Audigy drivers are listed under Soundblaster, select the 24-bit ADVANCED HD option.
The site confirms that all Audigy cards run on XP. Could be that the XP drivers weren't available when you got the card.
Can't guarantee that your PC will benefit from upgrading to XP, but I don't know why it shouldn't work, if it's only 2 years old as you describe. Whether it's a good idea for you depends a lot on how attached you are to your other applications, and possibly hardware peripherals which may not be XP compatible.