Totally Random.

[wpswart006]

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.

http://support.microsoft.com/kb/261186

Has it ever happend to you?

 

[chopsuey]

Yip, not sure about the tune name ....

.. only on very old systems, Pentium/PII

 

[requiem]

My last PC was so broken it performed classical concerts in several major cities