Disconnect all peripherals from your computer.
Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
Restart your computer when done.
Repair permissions after you reach the desktop-http://support.apple.com/kb/HT2963 and restart your computer.
Sluggish Finder - Bunch of icons on your desktop? An overcrowded desktop slows down your Mac.
Get rid of needless Finder calculations - Open a window in Finder/View/Show View Options: ensure "Calculate All Sizes" option is unchecked.
Internet related - It's likely that your broadband connection is the bottleneck. You can check your speed on http://www.speedtest.net to see how fast your connection is. If your web browser specifically performs slowly, quit and relaunch it.
If browser remains sluggish, empty its cache.
Safari/Empty Cache
Firefox/Preferences - select the Network tab of the Advanced preferences and click the "Clear Now" button in the "Offline Storage" area.
Application related - Launch "Activity Monitor" - Applications/Utilities - click the CPU heading and see what float to the top. If an application takes up a large chunk of CPU and won't let it go, it could be dragging down your Mac's performance. Quit it by clicking the Quit Process button at the top of the Activity Monitor window.
Too little ram - max out your ram if your computer can take more than 4gb.