Could it be that you're running TP full screen and the status bar is being overlapped by Windows' task bar? I guess not, but I had to ask :-)
When you're not running TP maximized, the lower-right corner of its window should show a small marking that indicates where you can pick up and drag the corner with the mouse to resize the window. With a correctly working version, this marking is located in the right end of the status bar. If you see that marking and you right-click your mouse slightly to the left of it, do you get the 'Customize status bar' dialog? (if so, and all items are Off, turn them On).
Some more exotic suggestions..:
Does the problem also occur when you 1) set your Windows task bar properties to 'always on top' and when you turn 'auto-hide' off and 2) re-start ThumbsPlus?
Are you using a Windows theme like Aero (rather than Basic), or, are you using desktop management software (e.g. for multimonitor setups) or customisation/'skinning' software, like Window Blinds? Are you running Windows in the native resolution of your monitor? Have you set a larger system font in your Windows personalization options? Are your video and mouse drivers up to date? (As you may guess from these questions I am looking for anything that might cause a discrepancy between TP's window size and the available space reported to it by windows, which might cause problems drawing the window and positioning its components).