Thanks for the replies. I suspect that Norton 360 is the culprit. I didn't
know how to "turn off" N360. I tried disabling the antivirus and the
firewall. That didn't work.
I have (had!) Norton Internet Security, so this might not apply to you.
In NIS, you go to Settings, then find the Miscellaneous button or tab. Look
for a setting to ... Oh hell, I can't remember. Hang on while I boot
another computer.
<Twiddle thumbs>
OK, I'm back. Under Settings | Miscellaneous Settings, in the area labeled
Product Security, turn off "Norton Product Tamper Protection".
Revert the setting after a successful System Restore.
This has worked for me many times, although often I remember only after a
failed first attempt at System Restore.
Since you have a different product, the exact sequence might be different
even if the setting exists, but the above should provide a clue, if we're
lucky.