Irritating blue rectangle

[Brian Norman Wootton]

I have an annoying dark blue rectangle which migrates from icon to icon 
along the
system tray, at the moment it's masking the volume control, but it 
covers all the others at some time or another. As long as I remember 
what's under the rectangle, the correct 'pop-up' opens up OK when I 
click on it, and sometimes it doesn't completely cover the icon like the 
time and date for instance. Any idea what is doing this?
brian