Many motherboards in old PC systems are still based on socket7. Users with such platforms often look for a cheap way to speed up their aged computer. And indeed, there are ways to replace a slow Intel CPU e.g. the Pentium 90 with a speedy AMD K6 processor. The advantage is that you don't have to replace the motherboard, thereby avoiding complicated mounting procedures. But there is another thing to consider. Almost all older systems are equipped with an AT chassis and AT motherboard. It is almost impossible to find motherboards that still comply with this ancient form factor. But, if you still want to undertake a full upgrade, you'd have to replace motherboard and chassis together with your CPU.

Good old Socket7 is still the platform that many old computers are based upon. Using fast K6 CPUs, you can do a cheap system upgrade without having to change motherboard or chassis.