I have a machine I built in 2000 around an ASUS motherboard and an Athlon Thunderbird 1GHz chip. That computer is still running like a champ and it stays on pretty much 24/7. It is my office machine, used for web design and maintenance, Photoshop, Illustrator, ripping music, etc.
I've replaced the power supply once and this past summer the video card died. Other than that, the damn thing just keeps going. It's extremely under-powered for most things I make it do, but I can't justify upgrading or replacing it until it finally kicks the bucket.