Curing Mal de Mac

Sometimes general or vague problems with a Mac - what I call Mal de Mac - can be solved by running Disk Utility and repairing the hard drive. But, you need to prepare properly. Disk Utility on your Mac will not repair your hard drive unless you follow the following steps:

If you are running OS 10.7 or later, restart your Mac and hold down the Command key and the R key (Command-R), and keep holding them until the Apple icon appears. After your Mac has finished starting up, you will see a desktop with an OS X menu bar and a "Mac OS X Utilities" application window. You can run Disk Utility and repair your drive from here.

If you have an older system, the best bet is to start up from your Install Disk. Again, you can’t repair the current startup volume with the Disk Utility that's on your Mac. You need to start up from a Mac OS X Install Disk and run Disk Utility from there. But many people, especially those who bought used or inherited an old Mac from a friend, may not have an Install Disk handy. Others simply no longer have a working optical drive, but soldier on.

If you have an older system, and no startup discs, here's another thing you can try:

Shut down your Mac in the usual way.
Boot in "single-user mode" by holding down Command-S at startup.
When you see the text prompt, type:

fsck -fy and hit <RETURN>

When it's done, type:

reboot and hit <RETURN>

Full instructions (and warnings) are found on this official Apple page.