From Microsoft support page: During normal operation or in Safe mode, your computer may play "Fur Elise" or "It's a Small, Small World" seemingly at random. This is an indication sent to the PC speaker from the computer's BIOS that the CPU fan is failing or has failed, or that the power supply voltages have drifted out of tolerance. This is a design feature of a detection circuit and system BIOSes developed by Award/Unicore from 1997 on.

Although these symptoms may appear to be virus-like, they are the result of an electronic hardware monitoring component of the motherboard and BIOS. You may want to have your computer checked or serviced.  For aditional related information, please see the following DFI Technologies Web site

Hah crazy ;-) Has anyone seen this actually happen? I'd like to see a video or hear an actual recording of a PC speaker doing this :-)