Buy Cheap PC (Custom-Built)

Buy custom-built PC cheaply with all the components you choose:
PCSpecialist

They do pretty cheap custom-built warranted PCs, shipped in carefully prepared shipping crates. You cannot choose the brand of the components, but for the price it's still pretty good.

If you don't mind building yourself, and want to get just the brand of component you want (e.g. Seagate hard drive, Antec dual ball bearing case fan, etc.), then you should try eBuyer.