Note, this portion only covers installing the motherboard itself.  You may also want to see HowTo Install A CPU. 

Why?

You may want to install a new motherboard for new technologies (AGP 4x, Ultra DMA 66), faster speeds (133 MHz bus speed), or just to support newer CPUs (Socket 370, etc.). Motherboard and chipset technologies can change pretty quickly, although the BX chipset has been a strong one for a long time now.

How?

1. This is the most complicated upgrade you can do, but it isn't brain surgery either.  It's kind of like wiping out the feet from under Windows. If you make this upgrade, you will usually want to *at least* reinstall Windows (just run setup again). It's probably best that you actually format your hard drive and start from scratch after installing the new motherboard, so you will want to back up your data, etc.

2. Of course, before you order your motherboard you want to be sure of a few things.  For one, the board should have enough slots for all your current cards (sound, video, etc.). Second, make sure it will support the CPU you have (or are getting).  Third, if you have an ATX case, make sure you are getting an ATX motherboard.
You will want to keep yourself grounded to protect your components by touching the screws on the back of your computer case or by using an anti static wrist guard.

3. Once you've backed up your data and purchased the proper motherboard, shut down your computer, unplug it from the wall, open the case, unscrew all the components from their slots and pull them out (if it's a tower, you may want to lay it on its side).  Unplug the keyboard, mouse, IDE and floppy cables, power supply, RAM, and CPU (and whatever else). There should now be nothing but screws holding in your motherboard.  Remove them and then the motherboard should lift right out. Lower the new motherboard in (make sure you have installed the CPU bracket if required), and screw it into place (you may need to use a different back panel that may come with the motherboard or case - where your keyboard, serial, parallel, USB, etc. ports are).  Plug in the CPU (& CPU fan), video card, RAM, and power supply cord - *nothing* else!

4. Boot your computer with only the basic components and make sure you have video display. If not, something's wrong with the few components you have plugged in.  If everything looks okay, plug in your keyboard and floppy cable (with drive attached).  Boot to a boot disk and make sure you make it to the A: prompt okay. If so, go to the next step - mouse, IDE cables. Make sure everything still boots okay.  Now you can try your other components like your sound card, modem, etc. (one at a time or all at once - it's your choice depending on how well everything looks so far).

5. If you're trying to go without formatting, just boot fully with the new motherboard, and provide driver CDs and/or Windows CDs when prompted. It's best if you restart every time that it asks to "Restart Now?"
If you chose to format, you may now want to move on to HowTo Install Windows from Scratch. 

6. That's all there is to it, but be aware that it often is not this simple.  You may run into problems with components or RAM not working in certain slots, not getting video or power through the motherboard, etc.  Often this is due to a defective component, but you may have something plugged in wrong. Unplugging each component and plugging it back in to make sure it is fully inserted can help, same with your power cable.  If available, having additional components to test in place of another can be very helpful in diagnosing problems.