This is not a joke, I repeat, this is serious business! I found this while surfing around on Digg, and thought this was one of the most interesting "features" I have ever seen.
According to Microsoft's Support website, if your computer starts playing classical music such as "Fur Elise", or even more modern hits like "It's a Small, Small World" this is cause for concern.
From Microsoft Support:
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.
They go on to say that if you encounter this problem, it is time to get your computer into the PC Doctor to have it checked out. Though it may seem like a virus, it could very well be a serious hardware problem.
- El Di Pablo