Are you using Vista? Right-click the installer & choose run as Administrator. If still not work, choose compatibility mode & run as win XP after right-clicking it.

Oh something else to try: http://www.gnutellaforums.com/333278-post2.html but of course, use your Pro installer, not the one suggested. Your computer 'might' also need a reboot. That is sometimes an issue, especially if having reinstalled Java.

If still problems, post back as much of these details as you can: System details - help us to help you (click on blue link)