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This comment troubles me. I have tested it on three pc's so far and it worked like it should.
Just tested, didn't happen on the AthlonXP, only happened on the Athlon X-2. On the AXP, the towers actually appears at the mouse down, before the button has to go up... going to try again on the X-2.

Like for example when you select the tower from the panel or the buttons in the menu?
Didn't notice anything in the menu/tower, will investigate more.

Are you using a special mouse or an unusual mouse configuration?
It's a Microsoft Explorer mouse.