There are no alt descriptions for boxes, plants, etc. but there should be for equipment, teleporter stones, potions, etc. If you hit the alt key then move the cursor over the item, the description should stay there; I don't remember every doing that the other way around.
Maybe it is a graphics glitch? As a wild guess, try running the configuration program and checking the 'Use AlphaBit' box. Have you checked for updated video card drivers?
Maybe a re-install (after backing up your saved games) would help?Troubleshooting Tips (& manual install procedure)