Clear off the desktop and put all those extra programs and files in the hard drive or an extra folder or two.
Click on hard drive, then on applications, then on utilities. First check out the activity monitor. This program will tell you how much ram each running program is eating up. Check out "kernel_task". This funky program is the master program for all other programs on Mac. Any program you open on OS automatically opens a shadow program on kernel_task that decodes the programming so that you can use it. The problem comes in when you open various programs and then later close them, the shadow program is still running on kernel_task. Kernel_task can become quite a beast, using up to 10% of the ram or more. The only way to fix this is to restart your computer and not start running any more programs than absolutely necessary.
Then go back to utilities and try "disk utility". Select the top icon above MacHD. Click repair disk utilities. Go find some tough meat to chew on while you wait.
that's the best I got, both those things have a good chance of helping, though may not do the trick.
