Go to http://europe.nokia.com/A4144790 and download the Nokia Internet Tablet Software Updater, follow the instructions carefully and your 770 should be as good as new once the installation process is completed. I believe I'm right in thinking that this is as near as you'll get to a factory reset, it's saved my skin a few times after unsuccessfully trying to boot from MMC.

It's also worth downloading the firmware separately from http://tablets-dev.nokia.com/ for 2 reasons: 1. It makes installation a lot quicker 2. Sometimes NITSU's connection times out and can't download the firmware, by saving the firmware to your hard drive you can simply point NITSU to the appropriate .bin file

Additionally you might want to re-format the MMC card, though it shouldn't make any difference.