Did you REALLY clear the CMOS?
This is just a quick guide on the best way I have found to ensure that a CMOS is reset.
This is also a good technique to ensure that you drain power from the system to reset any circuits that may have tripped out on a resetable fuse (for example network cards and onboard USB hubs.) If you just want to do this, and not clear the CMOS, then ignore the "Change the Jumper" section of the guide.
Cut the Power
The ATX standard is such that an ATX power supply provides a constant 5V to the system's motherboard. This is what allows it to come out of standby, perform a normal power up, and also shut itself off. This also means that even if the system is "off" it will still have power to it.
Ensure the system has no power by turning off the PSU at the switch on the back, or even better, turn if off at the wall socket.
Change the Jumper
As with any work inside a PC, observe anti-static precautions.
To clear the CMOS there will probably be a jumper on the motherboard. Your motherboard manual will help you locate it. Usually the clear CMOS header consists of three pins, two of which are shorted by a jumper. Moving the jumper from one pairing to the other possible pairing will usually drain power to the CMOS. Below is a picture of one of my motherboards, with the clear CMOS jumper block highlighted in red:
If you can't find it, or aren't certain you've found the right jumper headers, don't worry. Instead, just take the battery out!
Making sure the system is discharged
I find this particularly useful for ensuring the resetting of any fuses that may have tripped (I had to do this a lot with a network card that didn't like hibernation mode.) It should also ensure a discharged CMOS.
With the power to the PSU off, press the power button on the front of the computer. This will attempt to latch on full power to the system and start it up. Of course, it won't get very far, but you may get a flicker of light and fans spinning briefly. All power has now been used for sure!
Power up Again
Set the CMOS jumper back, or put the battery back in. Now turn power back on, and press the power button. Hopefully you'll be prompted about a checksum error, and asked to enter the BIOS setup. Away you go!
If you weren't resetting the CMOS, and were just checking to see if that dead onboard USB hub had just tripped out - then hopefully it is now working :)
