Ah. This is why you should always have spare screws on hand. Fortunately, there is a way...
Light Method
Most screws are silver. Shine a flashlight into the carpet and look for little silvery glints. This may take some time, but you'll probably find it. If your carpet is unnaturally thick...
Magnet Method
Take a powerful magnet (like a bar magnet, hardware stores - fridge magnets might work) and run it over the surface where you think the screw is. You should find it. If that still fails...
Vacuum Method
Take a good vacuum cleaner. Put a sock over the extending arm, and run it over where you think the screw is. The sock will prevent the screw from being vacuumed up, but it will be stuck to the sock. There will still be enough suction to pick up the screw, even with the sock. If not, get a new vacuum! If that fails...
Organize your screws
There are many useful tools to organize screws. Go to your local RadioShack and get one, or make one out of a magnet for that awesome lifehack feel.
Stop using carpet!
[Not a "find your screws" method - a tip about not frying your computer]
Carpet can cause static discharge, which is very very very bad for a computer (personal experience). Work on a hard floor with an antistatic strap to reduce the chance of frying your PC.