Strange beeping sound?

[garnet]

Hi
since yesterday there is a strange beeping sound every now and then. The sound doesnt come from the speakers it is more like the motherboard beeping sound, similar to the ones when in bios.
The pc works fine, but every 20 or so minutes I hear this beep.
Nothing is found in the windows log. The only thing I did was to deinstall office 2003 and install office 2007.
Restart doesnt help.
Working in win xp sp2.
Ideas?