One continuous beep after startup

Hello,
I'm working on a HP Pavilion a300n. On startup, I receive one long, continuous beep. At the same time, the power to the fans seems to drop in half because they slow down to about half speed.

The beginning of the beep sounds exactly like the sound you get when you hold a key down on the keyboard for a while. It's a very fast flutter. Then after about 5 seconds it turns into a continuous beep.

This is what I have done so far that hasn't corrected the problem:
New Power Supply
Changed RAM to known good modules
Disconnected all peripherals one at a time
Changed CMOS battery
Changed hard drive
Didconnected keyboard
Disconnected Mouse
Changed CPU from Celeron 2.6 GHz to Pentium 4 2.4 GHZ (both sockets 478)

I do get a HP screen on my monitor but it freezes immediately.

I've been working on repairing computers for more than 25 years and have never come across this problem. All that's left is to replace the motherboard (I think) but the system's too old to invest that much money.

Does anyone have ANY idea what the problem is????? I'm stumped.