If degragging a system fixes a problem other than program efficiency, you have something else wrong- and should probably perform a low level scan for hard disk errors.  If you have disk errors, there is a chance your hard disk is sick and could be on the path to hard drive heaven (or hell, if it was really bad:P ) Of course, all disks are born with imperfections, so don't write them off the first time. Just redo the scan a few days later to make sure additional ones didn't creep back in (make sure you told it to "fix errors")

Defragmenting files is kind of like reorganizing your kitchen pantry. You just scoot the cans of green beans together and put them where you expect to find them- it doesn't remove any dents in them:P