Discovered that when I closed FF 1.5, several firefox.exe's were still open. This prevented CCleaner from doing it's thing. Had to do Control-Alt-Del, and close all the other open firefox.exe's. Allegedly this happens because of antivirus programs or anything that monitors disk activity. Didn't have this problem with 1.07. 1.5 didn't seem that much faster to overcome this glitch for me.