I have windows xp profesional and every time i check my cpu usage one of the process's is taking around 89%-98% cpu. Its image name is System Idle process. I can't end it or set its priority.

This is not a problem. The system idle process is telling you that the system is in an idle state and not running any applications. All Windows XP systems have this process running. If you have access to any other machines go ahead and look at the process task list and you will see that the idle process is running at 98%. To check this click on the performance tab in the Windows Task manager and you will see the real CPU utillization is running 1~10%. On a rare occasion you will see the CPU shoot up for a very short time, it's taking care of house keeping function in windows. When you launch a program or have heavy CPU utillization you will see the idle process drop down. If you have an application that needs 100% of your CPU the idle process will be 0%.

Thanks