Hi Neil,
For ease of explaining, I have uploaded a screen shot here http://img57.imageshack.us/my.php?image=aceutils0ux.jpg
It happens with remove junk files, clean system registry, in fact any sub-program of Ace Utilities.
I have noticed it somewhere else too, but I can't for the life of me remember where that was.
I originally thought it was something to do with Dialogue backgrounds, but it happens whether the "Skin Windows Backgrounds" is applied or not. Hmmmm! Over to you...