Hxc since you have a usb drive you can actually run windows from that, you're going to have to tell the bios to read usb drives as harddrives though which if your alienware laptop is decently new you should have the option.

another thing you might try is to look around in the bios to see what the primary hdd is set as, quite possibly if you have it hooked up while trying to boot your laptop it might be trying to read the usb drive instead of your primary hdd and could be why it's actually not working as far as that goes. that is unless the hdd is completely undetected in the bios... in which case yes you would need a new hdd. having an older laptop hdd on hand is a good test but doesn't mean your other one is dead.

it could have possibly reset the drive priorities after replacing the hdd with the other laptop hdd. i would try to put your original hdd back in and see if it works.