iPad application failed to install from iTunes on Windows
Firstly, check to make sure that there is not a download in progress that has not been interrupted/paused.

Secondly, the file may be corrupt, my suggestion would be to remove the .ipa file from windows, and allow iTunes to re-download it.


right click the app within iTunes and click "Show in Windows Explorer"
find the .ipa file for your application and delete it.
re-download the app from your Purchased Apps page in iTunes.