Hello David-Cheshire,
Welcome to the Forums:)
I am sorry to hear about your installation problem with your Officejet L7780. I would suggest a more in-depth uninstall for starters to get rid of all of the files. After that is done, reinstall the printer.
1. HP Officejet Software and Driver Removal Utility
2. Restart your computer
3. Reinstall by using the HP Printer Install Wizard for Windows
I hope that helps!
Regards,