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.
I usually end up using something akin to two months before making my final decision.. and then a day putting the thing togehter - my previous com was still able to run most semi-modern games quite decently, despite the fact that it was severly aging.. custom build computers are so much easier to tweak and balance, because you know every bolt and bit in it.. and just how to push it to the edge.