It is true. This exerpt is from the Microsoft
support database, article Q261186:
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.
At least it is classical music and not the Jaws "shark's about to attack" music. I would like to wonder how many people have had to have medical attention due to being scared out of their wits when their computer started playing this music.