[Puzzled geek]

Windows 7 logs out within a minute or so of start up and returns to the logon screen, whether active or not. From there, the screen will also black out and re-initialise, I assume, the logon session, every minute, approx. This happens in both user accounts, with admin and standard user priviliges.
There was no recent software installation or other system change that prompted this. Windows updates may have occurred.
I verified the screen saver setting. I even set and cleared it to flush out any corrupt settings.
I rolled back several days of system files and settings, in succession.
MS Security Essentials, running in Safe Mode and doing a full systems scan, found and removed a virus rated 'severe': Trojan.js/Redirector.EV.
Windows 7 Ultimate, everything up to date.
A Google search did not come up with any advice. A virus with similar behaviour is described for XP only, and the remedy applies to XP only.
I assume this is a virus since I cannot determine any system setting or software change that could be the cause of this problem. How can I fix this?
I consider myself a competent user, happy to examine registry and startup settings safely.
Thanks, any advice greatly appreciated. i will continue by rolling the system back even further and employing alternative virus scanners.