Sometimes, starting your Mac up in Safe Mode or "Safe Boot" can fix some problems you may be having.
Safe Boot checks the hard drive for errors, rebuilds the font caches, and prevents startup items from running.
Mac OS X: Starting up in Safe Mode: http://support.apple.com/kb/ht1455
Mac OS X: What is Safe Boot, Safe Mode? http://support.apple.com/kb/HT1564
Safe Boot takes longer than normal startup: http://support.apple.com/kb/TS1884
NOTE: Even though Safe Mode checks the hard drive and it is possible to check the hard drive in Single User mode (with fsck), it is possible that you may still need to boot from disc to properly repair the hard drive.