I'm the same way. It's usually a full day going out and buying the parts, reading the motherboard configuration manual, and getting the OS installed. Still, it's a bit like riding a bike. It takes awhile for me to shake the techno cobwebs out, but after awhile, all the pins start to line up right.
I bought a store bought model once, and it was a disaster. It got too hot when I put a new video card in it, and it lasted only about 2 years as a decent machine, whereas my custom models usually go for about 5 with some tweaking.