1. Start your Mac in Safe Mode. Shut down your computer completely, then press the power button to start it up again. Immediately after hearing the startup chime, press and hold the 'Shift' key. Hold the key down until you see the gray screen with the Apple logo and spinning icon. Starting up in Safe Mode does a number of things, such as dumping program cache files and resetting them, performing a diagnostic and repairing errors and forces a directory check of the startup volume.
2. Restart your computer normally, without pressing any keys to see if the issues have been resolved. If you are able to log in, do so, and use your computer normally to see if you experience any issues. If there are still issues you may need to use the Disk Utility application with your Mac installation disc.

3. Insert your Mac installation disc, then shut down your computer. Press the power button to start the computer again, then when the startup chime sounds, hold down the 'C' key until the gray screen with the Apple logo and spinning icon appear. You should notice your CD drive becoming active, as it will be loading the data from the CD instead of your hard drive.
4. Select the main language of your choice, then click the arrow to proceed to the next step. This will not reset your system or wipe any files. Wait for the menu bar to appear at the top of the screen. If you are using Mac OS X 10.2.x through 10.3.x, click 'Installer,' then click 'Open Disk Utility.' In Mac OS X 10.4.x through 10.5.x, click 'Utilities,' then 'Disk Utility.'
5. Click 'Macintosh HD' on the sidebar once it has loaded, then under the 'First Aid' tab on the lower right, click 'Repair Disk.' This process may take a few minutes, but it will locate any errors on your disk and repair them. If there are no errors, you will see 'The volume Macintosh HD appears to be OK.' If you see this--and even if you don't--check for additional errors by clicking 'Repair Disc Permissions.'