The realtek audio driver(onboard) has somehow been corrupted by installing sp3.Yes, I downloaded the latest drivers and even uninstalled sp3 and tried system restore.Nothing!!!!.Also reinstalled the mobo drivers which came with it and still nothing.BIOS is not much help either unless I am missing something.I would have thought that reinstalling the chipset drivers from the mobo cd should have fixed it,obviously not!.

Win XP Professional sp3
2Gb mem
nvidia 8600gt gfx (onboard 6100)pci-e 16x
onboard realtek hd sound
AMD Sempron 2.8Ghz (AM2) socket

I'm seriously thinking about installing a sound card which i've held onto since 1993 to see if that works LOL!

Yeh! I'm trying to fix a friends pc,mine is ok I'm running Vista Ultimate x64.My friends pc is basically a complete rebuild from scratch and we're having loads of fun finding drivers to get it up to speed.I'll try what you said and get back here to let you know how things are going.I'm at my own pc at the moment so I can't do it straight away but we'll see :-).

mika, another thing I thought of is that some systems have a BIOS setting to disable onboard audio. So, check the Setup Utility / BIOS menus and make sure that it's enabled.

Did you check in the Device Manager? I would expect the Realtek HD Audio device to show up there, but if the correct drivers are not installed it might show up as "Unknown device". Do you see any question mark, exclamation point, or X symbols in Device Manager.