It's a different issue. If the phone starts up after replacing the battery, and will charge, then the battery is compatible.

Your assumption and oldturkey's response are correct. The battery will work fine as long as it has the same connections, and the same voltage (3.7V). Those capacity ratings (the mAh numbers) are almost always innaccurate and inflated anyway, especially when purchased from inexpensive sources.

16-bit Windows XP? That's not a thing. Another consideration for Bernard is that your E6400 may not have a license for Windows XP. There should be a sticker on the bottom of your laptop with a serial number, that's the version of Windows that you have a license for. You can use a Dell windows installation CD with that version of Windows. Definitely download the network drivers from the site Gary linked to, because you probably won't be able to connect to the internet on your E6400 when you first install Windows until you install the network drivers. Also, a friendly internet etiquette tip - don't type in all caps. It gives people a bad first impression of you. :)