My PC fans usually fail after 5+ years of use since I clean the dust off them on a regular basis and occassionally remove the sticker at the bottum of the fan to add a few drops of machine oil into them, so they spin freely.
My previous family-owned PC's cooling fan runs well, but the fan and heatsink was clogged with dust causing the CPU to shut down and eventually messed up the machine. What's strange is that the fan never increase its speed when the CPU temperature reaches danger levels. It was a Ultra (brand name) PC, and had a vent on the left side where air and dust gets sucked in by the fan. I'll never by PCs like that one again.
We brought a Dell Inspiron PC to replace the old one, fortunately this PC only has fans in the back pumping air out, and speeds up when the CPU runs hot.