[CentOS] Application icons don't appear in taskbar at bottom of display - they disappear out of bottom right corner!

Running CentOS 4.4 and something drastic appears to have happened with
my display setup - application icons no longer appear in taskbar at
bottom of display (when minimized) but disappear off bottom right-hand
corner. So it's difficult to multi-task because I have to keep re-
opening applications - can anybody suggest what I need to reconfigure
please because I can't find any obvious display setting which will cure
this problem - thanks.

Andy