The "Build Timing" key (not Build Time) is the only one at all under
HKCU/VisualStudio/9.0/VC (and actually the only key under the entire
HKCU/VisualStudio tree) on both my XP and Win7 new VS 2008 installation.
 I can't rule out it being part of SP1 which I included in the install,
though I found references on the web to it earlier than that.

It's strange though, because I've also verified that key on the older XP
build slave that was installed with VC Express (and never was installed
with the full version, nor did I use any service packs other than what
was part of VC Express), so I'm a bit surprised that you don't have it
in your install.

I don't have Fullscreen keys in that tree, although I did find a
Fullscreen key under HKCU/Software/Microsoft/VCExpress/9.0 just on my
express installation on XP.  It's possible that key only gets created
when you actually run Visual Studio (which I generally don't on
buildbots), but I suspect the path would differ between express and regular.

There's nothing else that looks viable under HKCU, at least related to
Visual Studio.  All of my other brand-new-install keys are under HKLM
instead, but there's a larger divergence between express and standard. 
It appears that on VC express most keys end up under
HKLM/Software/Microsoft/VCExpress/9.0, while under VS 2008 standard
they're the same path but "VisualStudio" instead.

Another choice that might be pretty safe as an alternate possibility,
but you have to be willing to separate the express and standard checks,
is the HKLM/Software/Microsoft/XXXX/9.0/ApplicationID key.  XXXX is
"VCExpress" or "VisualStudio" and that's also the value of that key.

Alternatively, as siblings to the ApplicationID key, I have two keys
that have common values between the express and standard installs:
"EnablePreloadCLR" and "RestoreAppPath" are both 1 in all installs.  I'm
guessing both represent boolean flags so accepting either 0 or 1 would
work, but that's a guess, so the ApplicationID key may be a better choice.