Does anyone know what causes this error message?

its a memory error, usually caused by some "locked" memory that the program was using but didn't "let go of" when the program ended, it can also be caused by partial install/uninstall of the same programs. I don't think it has anything to do with IE7, if you recently installed any other programs or made any changes to them that could be causing the problem. Check your add/remove programs list, and if it is IE7 that is causing the error, re-install it