|The first thing I would try is deleting the sound drivers that are already installed. This won't shouldn't hurt anything because you don't have sound anyway.|
Then reboot the computer. If XP auto detects the drivers you are golden. If not, then try:
On the desktop, right-click My Computer then > Hardware tab > Device Manager.
At this point you may see a device with yellow ? saying audio controller.
Right-click and "Update driver", and select Windows Update to search for a driver.
iMac G5 1.8GHz "Hero" PowerPC
2 x 1GB DDR PC3200 SDRAM
160GB WDC WD1600JD-40GBB2
Mac OS X Leopard Version 10.5.8