The two most common issues are something that installed itself in your Login Items and starts automatically running in the background or something that is just stuck on in the background.

For the first go to your accounts and check the Startup at Login Items to see if something needs to to be removed from the list and then restart the computer. The other thing is to go to the Activity Monitor which is found in your Utilities Folder. Be sure to use the drop down box for All Processes. It shows just My Processes by default and see if something is has your processor pegged when you’re not doing anything. If something has your processor continually pegged at 90% or more even though you’re not doing anything I’d say it’s something funky and you can just click Quit Process. I actually had to do this one fairly recently. Personally I’d still restart the computer afterward unless you know what the errant process is and know it’s not needed.