This is most likely because you have already installed a later Internet
Explorer cumulative update that incorporates the file in that update.
The update you are trying to install (KB818506) tries to update the file
Wininet.dll with the version number 6.0.2800.1207.
More about KB818506 here: http://www.yqcomputer.com/
Check the version of this file in your %windir%\system32 folder
(%windir%\system if you are running Win98/ME) and see if it is newer, in that
case, you don't need this update.