The major factors which determine the lifespan of a typical computer are dust and voltage through the CPU.
Dust is a common enemy which can be kept at bay by cleaning the PC thoroughly, twice a year.
Voltage through the CPU is well something no computer can get rid off. The current passed through each and every component wears it out eventually, no matter how sophistically we use it. But it's a good thing that it takes fairly long for a PC to die that way.
I have an 8 year old OEM still going strong as my HTPC with no signs of wears (like BSODs without any issues, freezes, etc).