After installing SP2 I noticed that Windows had replaced my Realtek HD Audio
driver with its own older version. It also now appears twice in the Device
Manager; I can remove one of them with no ill effects but it is redetected
at next boot and reinstalled.

So I removed both of them, installed the latest Realtek driver and rebooted.
Now I still have two instances - one (called Realtek High Definition Audio)
with the Realtek driver and the other (simply called High Definition Audio
Device) with the Microsoft drivers. If I force the Microsoft one to use the
Realtek drivers it crashes horribly with a BSoD (so I won't be doing that
again!).

Once again I can remove the Microsoft one with no ill effects but it's
redetected and installed again on the next boot.

Something in my memory tells me I always had two entries under 'Sound, Video
and game controllers' but the second one wasn't called "High Definition
Audio Device" and the Driver Provider was Realtek, not Microsoft. Trouble
is, I can't remember what it was called, or what it was for.

Anybody help? I could always do a System Restore to pre-SP2 and find out
that way but it's probably worth finding out here first whether I'm worrying
about nothing or not. It seems I've got two components of the same chip
with drivers from different releases.