Awhile back, Windows XP started doing the hardware auto-detect thing for no reason I could discern, and it reported it couldn't find "Microsoft UAA Bus Driver for High Definition Audio".

So I Googled around and found pointers to, but that turns out to be a really old hot-fix and it wouldn't install on my XP-SP3 because the patch was created before SP3.

However, there was an easier solution.  I searched C:\Windows for file names containing "hdaud" and found it in both C:\WINDOWS\ServicePackFiles\i386 and C:\WINDOWS\system32\drivers, so I just filled-in C:\WINDOWS\system32\drivers in the where-do-I-look-now field, and Windows accepted it.