I look for parts, I look for testing results, find a combination of parts that to my knowledge won't bugger eachother up and cause incompatability problems, and slap it together from individually purchased parts over the course of a month or so using sales on Ncix.com to build a decent rig, then I upgrade it periodically with more parts I get on sale for ridiculously cheap, and maybe build a new rig like I did the last one when the motherboard/ram/cpu needs a refit...
Currently I'm male, I'm 25 and have been building computers for 11 years, I went from a Tandy 1000 to an IBM Aptiva with a gaming gfx card and expanded ram(those upgrades were totally nonstandard, and I borked the warrantee putting them in, but I didn't care!) to a complete custom rig when I was 14 built it from the ground up from parts I ordered online, put a system together for around $700 that would have cost me well over $2200 retail, and felt so damn proud of myself doing it.