As jonny says try looking for nasties first.
However we had a joke program do the rounds at work. Someone with access to the computers installed the program and it randomly opens the CD. It didnt get found by antivirus programs. So see if someone you know is making a joke.
One script which opens the CD tray is found by my antivirus prog (which, otherwise, I've found to be a bit rubbish!) - depends on which one you're using.
It may also be that if you're part of a network (in work rather than at home), someone else may be doing this from another PC on the network. An antivirus prog wouldn't find this.