Join Date: Mar 2003
Location: Somewhere south of here.
The easiest thing to do and should be done once a week and whenever you add software is to do a permissions repair. You'll find it in Utilities / Disk utility. It will fix a lot of strange problems. For some reason, macs seem to get their permissions screwed up a lot.
As far as defrag goes, you can't really do a defrag on a mac unless you have an extra utility. Apple's site says defraging may not help and may actually hurt. Whenever you install an update and the screen says it is "optimizing the hard drive", it is actually defraging or compressing.