How can I add another processor to my Windows PC and use the existing as well as new processor simultaneously, essentially to boost overall processor capability?
There are multicore CPUs that you can swap into your motherboard, which allow for greater multitasking. However, any CPU that inhabits a single CPU slot is a single CPU, not a dual. While multicore CPUs are capable of managing multiple tasks simultaneously, a true dual CPU computer requires a special motherboard that has two slots for two different CPUs. These are frequently used in servers.
As far as I'm aware, the only way to add a second CPU to a standard motherboard is through clustering.A "cluster" can combine two computers to form a single entity and is frequently used as a low cost form of creating a supercomputer. There are Linux packages out there that allow clustering two completely different computers into a single unit. However, the process is note exactly user friendly.http://en.wikipedia.org/wiki/Computer_cluster
Unless your motherboard has two CPU Sockets you cannot ad a second CPU to your system.
Such boards are usually found in server systems or very expensive high-end systems. Since those motherboards cost several hundred dollars the chances of you having one without knowing is close to zero.