Dimension 2400 won't boot, Keyboard Failure, Steady Beep

 I have a Dimension 2400 that will not boot. At power up, a steady beep is heard, the monitor displays the "DELL" logo and progress bar goes about 1/5 and stops. Eventually it shows a "Keyboard Controller Failure" message. No lights are active on keyboard. The diagnostic lights on the rear are A,D- Green, B,C- Yellow and i cannot find documentation for this sequence. I have tried another keyboard and powering up with no keyboard connected and get same result. I'm thinking motherboard but was hoping someone has had this problem with documented repair success before i buy one. Thanks for any input or advice.