There are a few places that will sell you the barebones PC parts and then assemble the kit and burn-in for you. Or you can have parts only shipped only if you wish. You can get a PC with case, power supply, CPU, RAM, drives, etc., but without an installed OS, and take it from there.
Barebones options are good for first timers, because you don't have to worry about part selection as much. But you still have a bit of control. Here is one place that sells them. This is just an example, not a recommendation.
Warranty: Assembled and tested (1 year parts, lifetime labor).
Lifetime tech support: Customers who purchased a custom built product from AVADirect are entitled to free technical hardware-related support for the lifetime of their product.
http://www.avadirect.com/custom-barebones-desktops