OK, hope someone else is having this issue:

We're upgrading our ZfD agent from 4.1 to 6.5. On some Windows XP SP2
machines, the install looks like it completes, but on start up Zen never
loads. We've tried with the ZfD 6.5 agent as well as the ZfD 6.5 beta.
If we try to uninstall or reinstall we get a "Windows Installer Service
could not be accessed" error message. See Article ID 315346 on Windows
support site for all the fun fixes they recommend. I've been able to
find a "fix" for it using a combination of the aforementioned TID and the
Windows Installer Clean Up utility. That being said, both fixes were not
alike. i.e. I didn't do the exact same thing to fix them. Instead of
feeling like a kid banging on bric-o-blocks and just getting lucky, does
anyone have a real (works everytime) fix for this? Also, is this a ZfD
agent problem or a WinXPSP2 problem (I think I know the answer to this,
but figured I'd ask anyway).