Note that it is possible to assign running processes to certain individual CPU cores. So you could assign Sins to its very own core, which is what I do for my dual core CPU. I set the other programs, as many as I can, to the other core. You should be able to do this from the task manager by looking at processes and then right clicking on each individual process.
OMG Sanchezz posted something that is not directly advertising multiplayer!!!1 WHAAAaaa?!
And dirty, I guess you mean assigning a priority, and its simple as that?
I believe it's not what he meant. When you have multiple cores in your CPU and the application is not made for multiple cores, it will run on a single core only. If you don't tell the CPU which core should work with the application, the CPU will probably attempt to run everything on one core - slowing down the game. You can, however, tell the CPU to explicitly run Sins on another core, one that is not bothered with Windows and other.
Setting priority on the other hand is just telling the CPU to put this process above others. It's usually a bad thing to do, at least from my experience - I tried giving process priority with a few games and it usually caused erratic behaviour, like unresponsive keyboard and freezes (whereas keeping system processes AND the game all at a raised priority gave little results).
