How to configure blender so that it uses only 50% of total CPU when rendering? I need to render a very computer-intensive scene using cycles and for some reasons, I decided to use my home computer (i5 3.30 Ghz, 10 GB RAM, 2TB Hard Drive) to render it. However, the problem is that many people needs to use this computer. So, being generous and feeling responsible, I want to know whether there is a way to configure blender so that it uses only half of the total CPU. (Currently, it is using 100% all the time). 

Thanks!
While it's often more efficient to leave this to the Operating System, to answer your question specifically with a method that works, look under the Performance Panel in the Render Settings. It should be set to use the max amount of CPU threads by default, click Fixed and set it to half of however many threads your CPU has. It might look a bit different for BI and Cycles but the Threads part is what you want.

Obviously, you need a CPU with multiple cores for this to work. If you are on a single core machine, altering the value under Fixed will have no effect and your machine will also not use more threads that the value in Auto-detect.