Is this for a machine running a version of OS X that's 10.3 or earlier or is it for one running 10.6.8 as shown in your profile? If the latter then you've posted in the wrong forum.
Initially, try this:
Open Activity Monitor in the Utilities folder. Select All Processes from the Processes dropdown menu. Click twice on the CPU% column header to display in descending order. If you find a process using a large amount of CPU time (>=70,) then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that helps. Be sure to note the name of the runaway process so you can track down the cause of the problem.