If under the Performance tab in Task Manager it shows CPU Usage at 100% you have either-
Too many background programs running.
1) If you have more than 2 toolbars installed (see Programs and Features in Control Panel), uninstall any toolbars you don't need.
2) Limit any unnecessary programs that start when Windows starts. Start>Run type in msconfig and under the Startup tab disable anything that you installed that doesn't need to run constantly.
Malware at work.
Thanks to the hard work from all of the ******* that create malicious software, no anti-virus is 100% effective, and running two or more av programs can severely limit the performance of a system. I would recommend using Microsoft Security Essentials, it's very lite on system resources and does a pretty good job for a free av program (better than AVG or Avast). I'd also recommend using the free version of Malwarebytes. The free version isn't a real time av program, so it doesn't create a bottleneck with MSE (Microsoft Security Essentials), and, if updated regularly, if anything happens to slip past MSE, you can run Malwarebytes in safe mode to remove it. Malwarebytes is a VERY powerful malware removal tool, and their subscription based full time av program (Pro) is top notch.
http://www.microsoft.com/en-us/security_essentials/default.aspx
http://www.malwarebytes.org/products/malwarebytes_free
If, on the other hand, what you're seeing in Task Manager is a high percentage under the Processes tab as System Idle Process, that indicates how much of your processors ability ISN'T being utilized, or is Idle. It should have been labeled System Idle Percentage rather than System Idle Process, calling it a process tends to make the user think that it's an actual process that's running, instead of how much of your processors potential is idle. 100% in System Idle Process means that your processor is doing no work, or is 100% idle....0% means that it's working at 100%. Stupid, right?