Computer Virus Or Something Else What Can I Do To Stop It? Please Help.?

Well yea yesterday i was on AIM and som1 sent me this "I made this photo cube for you http://photoandmore.com.picsand.info/downloads/download.php?d=PhotoCubeViewer" and me being an idiot clicked on it and downloaded some crap. now all the time my internet explorer (which i do Not use i use mozilla) opens up with random pages all the time. and i receive IMS from most of my friends with the same message and some others IM me and say why did you send me this when i didnt at all . which is really strange what can i do please help?