Have you tried a Safe Boot. That can help by cleaning and adjusting some items on our Mac. Shut down the computer. Now holding the left shift key, hit the power button. Keep holding down the shift key until you see the login screen. Log in to your account. Allow everything to load. Next, shut down and reboot. See if that helps.
After that try using Onyx, which is a harmless program to tune-up and clean your Mac. It is made by Titanium, manufacturers of the anti-virus program I use. Onyx is free and I use it all the time. Go to: http://www.titanium.free.fr and click on the "download" tab on top. Download and install the version for your operating system.
After install, to to preferences and click on the update tab to ask Onyx to always check for updates.
When you run Onyx the first time, have it do the scans. Then click on the tab to clean the Mac (clean the user, system, internet, and fonts areas). It will take several reboots, but be patient. Next click on the maintenance tab. Rebuild the DYLD cache - doesn't take long. Repair the permissions and ACLs. (This takes a while - be patient.)
I would also advise you to at some point purchase Lion from the App store and install it on your Mac. That could also solve your problem. Snow Leopard is a pretty old operating system. The installation of Lion will also put a Recovery partition on your hard drive. That way, when you have problems, you simply start the Mac by holding down Command-R and it takes you to a set of Apple recovery tools. Lion is pretty inexpensive. I think it is only $19.95.