could be lots of things, caps on the board going, check your heatsink and fans on
cpu and gpu for dust, could be a heat problem. Could be grfx card on its way out.
I had an old p4 with rambus memory that someone gave me, powered up fine, no problem in the bios. installed windows and like you say, every time I moved the mouse it would screech from the small speaker on the mobo. Never did find out what it was, was too much trouble for something with 256mb rambus memory.