i have similar issue, also in quicklaunch but its the icons not repainting correctly.

This is using windows 7 with window blinds 7 (bought), and using Diamond skin.
Also happens with sabretooth skin, but worse:

And finally a downloaded skin, which looks terrible!

Look at my quicklaunch icons and system tray icons, its messed up! This happens with some downloaded skins also, seems to not be repainting correctly. Wish i knew about this before i bought it!
found a solution:
for my problem i was using windows vista basic theme instead of aero, so i unloaded window blinds, set windows 7 to aero theme then reaplied the skin. sorry for posting in this thread OP, thought we had similiar problems. you could try making sure you have aero set?