Bewildered

My computer was and may still be infected with the Adware.Agent.BN virus. After several days and numerous attempts to remove it with PC Tools Spyware Dr. i managed to get rid of it or so i thought. If i run Spyware Dr. or Norton i am told that my machine is clean at this time, however if i press Alt-Ctrl-Del and the Microsoft Security Dialogue Box appears, the button for “Task Manager” is grayed out and clicking on it does not result in a response.

This indicates to me that there is a program or file running that does not want to be detected. As i said running PC Tools and norton finds nothing. Do you have any clue or suggestions as to how i can deal with this problem?

Thank for your consideration.