Vista Reinstall of Vista Upgrade

Hi all
For some time my Vista Ultimate has gotten into a hang situation in which I
can not even raise the task manager. I presumed this was an early bug which
would eventually be cleared by updates. I usually restarted it by powering
up and it has ususually resumed ok. This time it resumed with severe
problems.. I tried several restores from past systems under my and under the
Administrator log on but each time it failed for 'an unknown reason.'

So I thought I would renistall the system by doing an upgrade. I could not
get this to run. The message was 'upgrade is not allowed because other users
are logged on.'. I am absolutely confident that this was not the case. I
logged everyone else off and even rebooted the machine and logged in only
one user.
Eventually I did a fresh build and even then it said I ought not to do it
because others were logged on. Is this a bug. Does the system log on
invisible system user logons which inhibit the upgrade. This was a very
severe nuisance
Roger Stenson