Inspiron 5100 Won't Recognize Memory Module

I would appreciate any suggestions people have on why my Dell Inspiron 5100 won’t boot with a memory module I recently bought.

Here’s the info: I currently have one 512 MB PNY memory chip and one 256 MB Dell chip in my computer. Bios version is A26. I want to replace the 256 MB chip with a 512 MB chip and max out on memory: the capacity is 1024 MB.

I bought another 512 MB PNY memory chip used on Ebay. The specs are the same as the other chips I have: PC 2100 DDR, 266 mhz. The PNY memory configurator part no. is the correct one for my computer. It is the same as the first 512 MB PNY chip, which works fine with the Inspiron.

I took the 256 MB chip out and inserted the newly purchased 512 MB chip in the slot. I made sure it was firmly seated. I booted. Or tried to boot. I got blinking lights on the keyboard and then the machine shut itself off.

I switched the two 512 MB chips in the memory slots. Same result.

I took both chips out and inserted only the recently purchased 512 MB chip in one of the slots. Same result.

Each time I checked to make sure the module was firmly seated. I’ve now put both of the "original" modules back in so I can use the computer.

At this point I was ready to conclude that the newly purchased module doesn’t work. But my wife’s laptop, an Apple Powerbook, has the same memory specs. So we put the module in her laptop. Result? No problem, the Apple boots up, and recognizes the memory. At my request she ran a hardware diagnostic test using a utility that came with the Powerbook, and it says the memory is good.

So I don’t get it. I’ve got the right memory specs, I’ve got memory that works, and I’m not exceeding the Inspiron’s rated memory capacity. Any idea what’s going on?