The first thing to try is repairing permissions with Apple's Disk Utility, which is located in the Applications>Utility folder. Launch it, and then click on the icon of your hard drive, and select Repair Permissions (don't bother with Verify Permissions).
There could also be some bloated caches, fonts in particular. If you turn your computer off, there are also some routine maintenance scripts that the OS runs which are not being run, because they generally take place very late at night (provided the machine is on, not sleeping, logged out or shutdown). If you would prefer not to leave your machine on all the time, there are several free and/or shareware utilities that will perform these functions for you. They're referred to in this article
from Macworld. I use one called Tweak Freak.
Hope that helps. Keep us posted.