Have you tried to completely uninstall DisplayLink? Not just via Programs and Features but using the DisplayLink Cleaner Tool you download from the DisplayLink site? The cleaner tool is here:
http://displaylink.com/techsupport_f...ll_cleaner.zip
If that does not allow you to reinstall 7.4, then run it again and uninstall your graphic drivers. Perhaps using something like Display Fusion, after using Windows uninstall, to make sure everything is deleted.