IF YOU OWN A CAR, you know what happens when you neglect little things like tuneups.Performance starts to deteriorate, so slowly at first that you hardly notice it. But after a while, the car doesn’t go as fast up hills. It gets noisy, and pulling onto the expressway in traffic becomes an act of courage. Eventually, it may not start at all.Your computer can stand an occasional tuneup, too.Its main moving part, the hard disk, may be the most important part of your machine. It’s where you store your programs and data files, which is reason enough to keep it healthy.