Registry Editing Hosed My System: cannot run system restore

I managed to hose my XP Professional SP2 desktop system by some global
permissions changes to the registry.

Windows appears to boot part way now, but generates just a blank screen with
my desktop background color and desktop photo, then stops. There are no
desktop icons and no taskbar nor start button. It does not respond to the
keyboard Windows button, and there's no mouse cursor.

Booting into safe mode generates a similar situation, with just a black
background and the usual Safe Mode text across the top of the screen.

It appeared that KB 304449 addressed this issue, such that I could do a
restore to a recent good point, but when I attempt to run rstrui.exe from
safe mode command prompt, nothing happens on the first try, then it opens
with a completely blank windows on the second try.

I see that KB304449 says to logon as an administrator, but I don't know how
to do that in safe mode command prompt. I attempted a runas to access the
program as administrator (runas /profile /user:Administrator "rstrui.exe",
but it responds with Error 1084, "This service cannot be started in Safe
Mode".

What should I do to get System Restore to run from the safe mode command
prompt, or should I be trying something else entirely?