can only log in as root

Tried to boot from a Windows hard drive attached to a USB cable today- it didn't work, and coincidence or not Xubuntu (12.04) got crippled in the process. Couldn't log in at all, it just kept rebooting. I restored the old passwd file (the one the system had renamed passwd-), and now I can log in, but only as root. Have looked at all the other threads listed by the google search and none of them help. Have tried to create a new user (successfully), but then can't log in using it either- still only as root. Would quite like to be able to login as the user I could log in as this morning before Windows cursed my computer. Would also like not to have to reinstall the OS if possible, though that is an option provided it isn't going to ruin everything else.

Cheers!