I've owned an Alienware m9750 for a few years and yesterday started having an unusual problem with it. I somehow contracted a svhost root kit virus and clean it of the infection. After getting everything back up and running I ran safe mode and ran the Windows 7 repair tool. It didn't find any problems but when it rebooted neither the keyboard or the touchpad would work. I couldn't even get the Bios to load or Shift-8 to get back into the safe mode menu during boot. Both a USB keyboard and a Bluetooth mouse work fine in Windows but the onboard stuff is completely unresponsive even though it had been working fine moments before. My gut tells me something in the motherboard went wonky but before I look into replacing it I wanted to check to see if maybe I'm missing some obvious other option.
I have seen this before as a software issue NOT a hardware one. The virus attaches itself to files that once removed disable the drivers of the keyboard / mouse connected at the time. Can you use the built-in keyboard to get into your bios? If it works then you know its windows. Also try looking in your device manager for drivers with a "!" next to them with the usb keyboard / mouse that you say work.
If it is windows try using system restore to roll back to when you still had the virus. If that fails then you may have to re-install the OS.
Again I have seen this happen MANY times after a virus removal. Unfortunately the only fix seems to be an OS re-installation as I have never been able to repair the drivers.