There’s been a fair amount of articles written on the web regarding what can happen with certain spyware infiltrations which result in the immediate logoff effect. Such articles as this discuss various fixes for the problem but don’t show the way I managed to fix it earlier this week.
Basically after I’d run a spyware scan using SpyBot Search And Destroy I still couldn’t login. It turned out that the file mentioned all over these various articles, userinit.exe, had actually been deleted in my case. I booted up using the Ultimate Boot CD for Windows, copied the userinit.exe from there and it all ran nicely.
Maybe that helps someone.