This morning I spent an hour on the phone to Microsoft support. I tried to log into my machine only to see the error message
The user profile service failed the logon.
User profile cannot be loaded.
The guy on the phone got me to put my Vista install DVD into the drive, boot from that, go to system-recovery, and revert to a restore point. Worked nicely.
The thing is, why did this happen? The only thing I can think of is the open source Firebird DB server. I installed it on Sunday to get an old app to run (installed as app, not service). I didn't install the control panel plugin due to a warning on the site telling me not to install it on Vista or it will trash my Control Panel.
I suppose the warning about the control panel should have been enough to put me off. Still, it works now. I just wont be installing it again, at least for some time.