rosshomer:First, I reset the CMOS, not the BIOS. That worked once. The computer will NOT boot with ANY USB device plugged in, in any USB port. I can, sometimes, plug in the keyboard/mouse when it's done booting. But not always. There is obviously something wrong with the USB system. Also, the same thing happens if I use a USB adapter and plug into the old-style round keyboard/mouse ports. Remember, this XPS is only 13 months old and I've never had a problem with a Dell product like this.
Hello rosshomer, Thanks for the update. The CMOS and the BIOS are essentially one and the same. The CMOS is the physcial device (chip) and the BIOS is the data (code) stored in this device. When you reset the BIOS, you reset the data (code) in the CMOS (chip).
How did you reset the BIOS? Did you remove the onboard battery on use the jumper pin method?
Either of these methods usually sets the BIOS back to the default settings.
It may be possible that your onboard battery has become faulty and needs to be changed.
Since these are cheap and easily acquired, and replaced, I would replace it with a new one, and not a used one.
This may or may not resolve your issue, but this would be the easiest place to start, as it may require a process of elimination to resolve this issue.
The keyboard and the mouse are on the same buss, so if one does not work because of a buss fault, the other would not work either.
You also mentioned that the other USB devices did not always work either.
This points toward the USB buss which is controlled by the chipset and chipset drivers.
It is possible there could be a motherboard issue here that results in the USB buss fault, but I would rule out all the other possibilities first.
Hope this helps.