If that also doesn't work, try resetting the warnings in iTunes' preferences:
The user-profile location is different, (XP: c:\documents and settings\username, Vista: c:\users\username). If you backed up your playlists and such from iTunes on your Vista system, it could be that your iTunes folder path is not set correctly here. While you are in the preferences, you might as well check that, and make sure the folder exists.
If there is an issue with the files, after the warning reset, iTunes will complain if there is something wrong when you try and add them.
Something else to try would be to connect your iPad, then in its settings-page in iTunes, enable "Manually Manage" and then rather than trying to add your videos to iTunes' Library, drag them directly over to your iPad's movies folder, and see if that works.