Mac won't accept user login

I was running out of disk space, so recently wiped my hard drive to eliminate a windows partition, and reinstalled. I was running Mountain Lion, bought as a download, and had problems reinstalling, and finally succeeded by using a friend's Mountain Lion disk, and reloaded everything via Time Machine.

I had two user accounts, one main one and a spare, both with admin privileges. Now it won't accept the password for the main user account.

I've actually managed to reset the User password using my second user account; I've tried a new password, complete with new password hint. But when I key in the new password, it doesn't accept it. It seems to take a couple of second before I get the reject wobble.

I know the new password has been input because the reminder has changed. But the login window simply won't accept it. I've tried a couple of variants, including simple words and all digits and whatever happens, it gets rejected. No, I don't have the caps lock on!

On apple's own guides, there is this:

OS X Mountain Lion: Reset a login password

But it doesn't work - I don't have a question mark in the password login field, simply the blinking cursor. What I've been trying, above, is the final suggestion, but again no dice.

It's probably irrelevant, but I'm also getting weird errors using the secondary user account; it's telling me iTunes won't work with Mountain Lion, for instance, even though it worked before.

Thanks in advance for any suggestions.