Right hand top used to show date & time, network, etc. Does not show these anymore. I tried to fix this in system preferences - date & time, but it just don't work!
How much free space do you have on the hard drive? If it's over 2 gig, then I'd suspect corrupted preference files.
If deleting the appropriate preference files and restarting doesn't resolve it, then (and maybe even if it did resolve it) I'd suspect drive corruption.
Preference files to check ("~" means "in your home folder"):
~/Library/Preferences/com.apple.MenuBarClock.plist
~/Library/Preferences/com.apple.iCal.AlarmScheduler.plist
~/Library/Preferences/com.apple.systempreferences.plist