Try a System Restore to the most recent restore point before the problem began.
Thanks for your reply. I already tried that; didn't work. For some reason (perhaps unrelated), system restore never works for me. Whenever I do a system restore, I always end up getting the message that the system could not be restored to my selected checkpoint, and I should select another. This happens even if I select one of the systems own default checkpoints.
However, I have to mention here that the problem sort of went away on its own last night after I had posted the message asking for help. It was very weird and I am not at all sure that it will not come back. For the past few weeks (or perhaps a month), I noticed one strange behaviour - upon regular shutdown, there would be a dialog box coming up that said on top "trying to close TRd:ww..." (the rest of this line went beyond the width of the dialog box) and there was an END NOW button. On pressing it, the computer would shut down normally. I tried to research this issue, but not having much to go on, did not find anything. Of course, I ran A/V and Anti-Spyware programs afresh, but did not detect anything unusual.
Last night when I tried to shut down the computer, the same dialog box appeared. However, this time, there were two other dialog boxes that popped up with error messages, that had "GINA UI" written on top. When I clicked OK to those message, I got a final message that the GINA UI and Winlogon had crashed. The computer shut down normally, and thereafter, my original duser.dll problem went away completely!!! Now I don't even get the strange "TRd:ww" message, nor any GINA UI error message.
At the Microsoft website, GINA UI is defined as a shell for Windows logon (I did not read thoroughly). I have to learn more about this, and that may give me a clue as to what really happened.