i have had lots of computers and have built many for other people friends and family, here are a list of parts that i have had fail... (this has been over about an 8 year span with about 30 different machines)
1 X bad powersupply(it was a cheep case powersupply that came with a case it lasted about 1.5years)
1 X bad motherboard(i just replaced this about 2 weeks ago, it was a computer made for a friend) bad capacitors
Leadtek K7NCR18D PRO(I have built 2 others with the same motherboard I still have one and another friend has the other I inspected the caps on those motherboards and they are fine, I use one of them as my spare/backup computer)
2 X bad pioneer slot loading dvd-rom drives, they worked for about 1-2 months then started having all kinds of problems, including the drive actually scratching disks(upon ejection)..
1 bad memory module (512MB G.Skill TCCD) this was bad from the factory, returned it for a replacement and im using it right now to type this...
2 x IBM Deathstar hard drives 45GB(the worst hard drives ever created!) sheesh...
now this isnt talking about just one computer, its a span of about umm 25-30 all home built.