You'll need an operating system and a processor
*He'd probably only use win7 home as he probably won't be utilizing the features in pro
and ultimate such as xp mode.
*I recommend an i5 if you have the money or otherwise an i3 will do just fine for gaming.
Hope I helped!
EDIT: I forgot also you will need a psi(power supply unit) and a hard drive. If you are still confused check out this video tutorial: YouTube - Broadcast Yourself.