This rig is pretty beefy.  It's been really stable too.  Except for
HL2/SOurce stuff.   Turns out, it's audio acceleration that has been the
problem.  I've been running HL2 on this X2 rig from git go with no problem.
HL2 and Source were fairly mature by the time I built it.  I'm using the
onboard sound chip and the most recent driver update from nVidia for audio.
Just recently, like the past month, I've started getting frequent CTD's.
Well, at first they were total lockups.  Reset button.  After a few of those
(in 1 hour), XP Pro x64 starts giving me the "We're sorry, HL2 has
encountered a problem and needs to close." errors instead of total lockups.
Then the error reporting starts giving me links to more information.
Following up on that in the Steam forums, I go through a short list of stuff
and decide to look further into audio looping crashes.  Cause that's what
the total lockups were doing.   Best advice I saw was to turn audio
acceleration down, a notch at a time, until the CTD's quit.  With the slider
down to "most basic acceleration only", the CTD's stopped.   So, why after
almost a year does this start up huh?
Ok, now I'm also finding issues with server list not adding servers I've
played on to the history, nor will it keep servers in my favorites.  Also, I
note that lag is hideous, even when I have a latency of teens or twenties,
lag is atrocious!  This is new.  Never had a bit of this in servers with 2
digit pings before.
So what's up with Steam now?