There are processes running that take all 100% of the CPU. Its called "pegging" the CPU. It can be a badly performing program, driver, or background process / service. You can usually see this in the task manager (how much of the CPU its using). When the CPU hits 100% the system freezes until the CPU can get some cycles back to work on its tasks.

http://www.windowsanswers.net/articles/fix-cpuusage

It could potentially be a badly written driver or some background process but i would start with shutting off the things that you don't need. For example, when you run Adobe Reader it puts a program in startup memory to update itself, same with Java, same with Google Chrome, same with iTunes, etc. If you go to the start button, type "msconfig" in the run box and look at the Startup tab you will see some of the things that are running. These are the background programs. you'll also have a number of services as well. While you can turn them off or disable them you need to be careful as some are required for system operation. So just dont go willy-nilly turning things off.

But also take a look at the task manager and see if you can see what programs are taking up the 100% of the CPU and figure out if you can turn that off.