I built a mainstream gaming computer and fit it all in a box I bought for $8 at the art store and you can do it too!

Step 1: Gather Your Components

The parts of my computer include:
Mother Board: I chose a mini Itx form factor because of it size. Zotac g45itx-b-e
A CPU: Core 2 duo E7500 2.93GHz
Ram: ddr3 2gb 1333mhz (any ram the is compatible with the motherboard is appropriate)
Graphics card: Nvidia gt240(needed little power so I could save space with a micro Itx power supply)
CPU Cooler: Zalman CNPS7000 ( looks fancy and works well)
power supply: 450watt micro ATX form factor
Hard Drive: I chose to use a laptop hard drive to maximize space (2.5" hard drive)
Computer Case Front Panel Wire Kit: This is very important as the computer would not turn on without one.
The Box: (30 x 15 x 10) or smaller should do not too small, you could even use a brief case(if you do please let me see!)
maybe some LEDs to brighten things up.