i have this problem with the interfaz of Arduino
here is the screenshot
Mine used to do the same thing on my vista pc, i wound up formatting that pc to switch from 32 bit to 64 and when i reinstalled it was normal, the main pc i use for arduino is a laptop with windows 7 and its never had problems.
I don't know what the problem is and i cant remember if reinstalling it was what fixed it for sure but just try uninstalling and reinstalling arduino and maybe try a java update.
I came across this thread today. http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1255462031
It seems that its a java issue update your version to hopefully fix it.
Fixed, Gracias !! 8-)