Having some bizarre trouble at the moment..features are literally disappearing from various programs!

First I couldn't get the customize toolbar pallette to appear in Safari and now Instant Alpha (and anything else in the select drop box for that matter) has vanished from preview! And I know it is only applicable to some image types etc, but its not available for anything. And even Preview help doesn't have ANY topics on Instant Alpha!

So yeah, what on earth is going on! Since I've got my iMac its been superb in that it doesnt crash even 1% of the amount any PC ever has....but if instead it just randomly loses program features I'm a little worried!!

Please help!!