Fixing drives w/fsck
fsck sounds dirty, but in some ways it is a life saver. Boot into single user mode by restarting and as your Mac boots up hold down the Command Key and the S Key. The screen will come up black and white like your on a terminal in the 80's. After everything scrolls up you will see a command that says something like /sbin/fsck -fy. Type it exactly as it appears including case sensitivity and spacing. Running this will do repairs on your hard drive. If you get any other message, other than your disk appears to be fine, run the fsck script again. When your disk appears to be fine, type reboot and your computer should boot up normally into the familar Apple GUI.
The other thing to consider is a cheap fire wire drive. You can use Carbon Copy cloner to make a copy of your system on to the fire wire drive. Then go under system preferences, choose start up disk, and specify your external drive and restart. Now you can use the disk utiltiy program on your external hard drive and fix your internal drive and you can back up your files in case something does happen to your hard drive.