Try removing all the sound card drivers from device manager and then reboot. They should come back. If not, reinstall from either ASUS's web site or from the CD. This should fix your problem.
My guess is a simple corruption of drivers.
It isn't a sound card but an onboard sound chip on my motherboard. And I very much doubt it is anything software related - atleast when it comes to drivers - because this problem even carried over after I did a fresh install of Windows XP about a month ago when I got a new hard drive.
By fresh install you mean you put in a new master drive and formatted it etc...? I will assume so.
Does ASUS have anything to say on the subject?