Martin:
I ran sfc and it executed with no errors ("Windows Resource Protection did not find any integrity violations").
Then I used msconfig.exe to disable all non-Microsoft services, as you directed, and did a restart -- no improvement -- the system behaved the same, pausing on an empty blue screen.
Here's more info that might be helpful: During boot, the usual things happen up to the display of the Welcome message. During the display of the Welcome message, the disk rattles as usual but then slows down to just a few flickers. After a few sectonds
of this, the screen changes to empty blue. After a few more seconds, the disk starts rattling at 'normal' intensity and the blue screen clears (though on one occasion, as I described above, it stayed on the blue screen).
Maybe during the pause in disk activity during the Welcome display the system is tring to access the internet? And getting the root cert problem? While waiting maybe the Welcome times out and blue screen comes? Then the internet reply causes the resumption
of the usual boot sequence? (and if no reply, no resumption?)
Anyway, it looks like a Microsoft problem, since the problem seems to occur even with just MS services enabled on a 'clean' boot. Has anyone checked that the certificate it wants is still valid? That looks like the obvious thing to check.
Looking forward to your reply, and thanks for your help.
Addendum: When I went to resture the normal boot (using msconfig.exe) I found TrueVector was enabled, which is ZoneAlarm's firewall, in case that's useful info. Also, my hardware is a Dell Inspiron 531 in an off-the-shelf config.