The answer to the question really depends on what you’re doing at the time. If there is a lot going on then it stands to reason that the processor is busy. Notoriously CPU intensive programs like video encoding, moving files, downloading, or games will use up a lot of the processors time, so it should come as no surprise if the CPU is at 100% while running them.
If the computer is near idle and the CPU runs at a high percentage then there is something else going on. Using Task Manager should tell you what’s going on, but it might not always know.
There are also things called software loops, which are times when a program gets caught up in something and can’t let go, a bit like a dog chasing its tail. This can be caused by a variety of things like badly written software, programs with memory leaks, or a perfectly good program caught up in a Windows page file error. There are many causes of these kinds of phenomenon and they are fortunately fairly easy to address.
Using Task Manager, find out what the program is that is taking all the processor time and highlight it. Then click in the bottom right, End Process. This forces the program to shut down immediately, and your system should return to normal. If the program is okay, restarting it should allow it to run normally again. You will loose unsave data though.
Unfortunately the most common cause of a busy processor on an idle machine is a virus. They hide themselves, or appear as an unobtrusive program which is why they don’t very often show up in Task Manager. The best thing you can do in this instance is stop what you’re doing and run a virus scan. Once the virus scan is complete, run a spyware scanner, then check for spyware.
Nothing should really cause a processor to run at 100% all the time. If yours does, then following the steps here should allow you to identify and address the situation.