Here is a handy Checklist you can print out to help you make sure you completed all of your steps properly.

Make sure ALL your jumpers are set correctly. (If you have a jumperless motherboard make sure to do this when you first configure your BIOS)

Check all your cards and make sure they are seated properly and fastened to the Motherboard with a screw.

Check all wires and plugs to make sure that they are well connected. A loose plug can result in data errors.

IDE cables and power cables for both CD-ROM and hard drives should be secure and properly installed (The red stripe on pin 1). Check CD-ROM and hard drives to make sure the jumper settings are properly configured depending on whether they are a Master or Slave.

Make sure the IDE cable for the floppy drive is not installed the wrong way. (If it is you'll get a steady green light) Make sure the power supply is properly connected.

Make sure your memory is installed properly with the tab securely in the holder.

The CPU should be well seated in the slot/socket with the cooling fan/heatsink assembly firmly attached. The cooling fan power cable should be connected to the power supply.

Your ATX power supply should be connected to the motherboard.

The connectors to the Power switch, Reset switch, Power LED, HD LED, and speaker should be connected in the proper place as recommended by the motherboard manual.