Two years?
Although my computer runs 24/7, I guess I am what you call a sunday driver.
I am careful with my PC as I am with my cars and the only problem I have had in the past is HD memory, (this is not counting bad OS like ME). Overcrowding the HD has always been the performance bugaboo for me.
I am not sure what you have to do to wear out a PC in a year or two but I obviously don't do it. My current box has a 400gb internal HD and a 100gb external so I am fairly confident it will last until the next generation tech becomes impossible to resist.
The PC that I replaced is 8 years old and is still working fairly well. I hope to get a lot of years out of my current box unless, as I said, the new tech become irresistable.