I am using a Windows machine with 8 cores. No other process than Mathematica 8 is using CPU time right now - still the MathKernel.exe CPU usage does not go beyond 13%. My current computation takes forever and I feel that might be the reason. How can I adjust the CPU usage of Mathematica to use, say, up to 90% of CPU?
"% of CPU" represents the entire capacity of your system. If you have N cores and a process uses only one core, then it will use 100/N % of the capacity of your system --- you obviously have 8 cores.
Unless a problem is explicitly parallelised, it won't run on more than one core.
You should be able to parallelise Simplify / FullSimplify:
Simplify[ParallelMap[Simplify, a + b + c]]
It may or may not save you any time.