BleepingComputer.com: Dell Dimension 5150 fails during POST

I'm working on a Dell Dimension 5150 that doesn't boot far enough to display messages on the screen. The lights for both DVD drives turn on briefly, and the hard disk light turns on for about a minute. Meanwhile, diagnostic lights 3 and 4 turn steady green and then the computer beeps continuously every half second.

The manual says that the diagnostic lights mean a memory failure. However, it does not mention the beeping. I've placed both memory modules into a different Dimension 5150 that recognized both. I've replaced them with the ones from the working computer, and gotten the same error indicators. The beeping doesn't match any beep code patterns I've located on Dell's Web site or the Internet.

In addition to replacing the memory modules with known good DIMMs, I've tried: Cleaning the memory modules and slots with contact cleaner; substituting the video card from the working computer; installing a new power supply; replacing the battery; and resetting CMOS with a jumper.

A POST diagnostic card showed 45 and then stopped with code 88. Unfortunately, I've lost the manual for this card, and I'm not even sure if those numbers are decimal or hex.

I suspect the motherboard needs to be replaced, but since Dell wants over $200 for a refurbished one, I'd like to be sure this is the problem, and not, for instance, the CPU.

Here are some details from Belarc Advisor:

BIOS: Dell Inc. A05 03/31/2006
CPU:3.40 gigahertz Intel Pentium 4
Display Adapter: RADEON X300 SE 128MB HyperMemory

Any ideas about what's wrong, or suggestions for what to try next?

You can get a new one from Newegg.com for less than 60 bucks. You will need new DDR2 memory though. Its still less than 200 bucks. I have the biostar it works perfect.

Edit: I would try removing everything (accept the memory and CPU) and seing if it will post then, If it doesn't order a new MB.