Use the task manager (in 2K/XP) or the Ctrl-Alt-Del Programs list (in 9x) to see a process called either b.exe or av.exe. Kill that process and delete the file, which should be in your C:\Windows or C:\WINNT directory (whichever is appropriate). Then use regedit to search for either the name of a key called "antivirus" or search for a key that references the same name of the executable that you deleted (IIRC it should be in something like HKLM/Software/Microsoft/Windows/CurrentVersion/Run). Delete that key. Then, use your AIM client to remove that link from your profile.
