PC Desktops and Laptops come with a variety of processors, video cards, memory, hard drives, etc. All of these components make a difference in how long your PC is going to last. You should get at least a 3yr minimum to a 5yr average lifetime on your PC. However, purchasing the cheapest PC will often come back to bite you with a shorter than average lifespan.
Changing out a PC, including moving over all your files, reinstalling programs, printers, drivers, etc. is a hassle. The PC deals listed on this site are hand picked to get 5-8+ years out of your investment. You pay a little more up front, but trust me, it is worth it in the long run.
You also shouldn't pay a premium for the newest processor on the market. Like new cars, the latest components lose their value quickly soon after the next new thing comes out. I recommend purchasing the more expensive components about one or two generations behind so that you get the most value.