Logon Script registry check how to?

We have a custom key set in the registry of our XP client machines to
differentiate our image build versions. We would like to be able to check
that this key (or variants of, depending on build number) exists when a
users logs on to the domain (to be sure they are using current builds or
even actual internal images).

Can this be done via a simple logon script where it would check the presence
for one of the defined variable in the client registry (ie:
"\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\S ession
Manager\Environment")? If the variable is not present it would be logged to
a .txt file. I imagine this wouldn't be too difficult but my scripting
skills are simply not up to the job...appreciate any feedback.