Haven't touched a Mac pretty much all summer, so I'm out of practice . . could be any number of things. Did you recently install software that uses its own installer? Some poorly-written, third-party installers have been known to change permissions on System-related folders, then neglect to set them back, causing problems after the installation. I suggest jumping on the forum board over at spymac.com. They've been a great help for me over the years.
This is hindsight, but it can't be said enough times . . back up your files frequently, 'coz it'll always bite you in the ass at the wrong time