Macbook says password is wrong, but it isn't.

There are three options I can think of. First, and most likely*, your girlfriend is confused and is not entering the correct password. Second, something is very wrong with the system or the account. Third, someone has reset the password on her account in order to get access to her data (or just to prank her). Try resetting the password:

http://support.apple.com/kb/ht1274

* Note that I'm not saying she's confused because she's a girl, before someone misreads that! It's just that, most often, in my experience, when someone says their password isn't working in a case like this, they've gotten confused and forgotten what the password is.

A fourth thing I can think of is that a different keyboard layout has been selected and it is remapping one or more characters in her password to a different key. I got caught by this once when I accidentally switched to a Hungarian keyboard layout - it moved a letter in my password to a different key!

you'll need to change the password to log in...here it is how..

insert the mac os x installation disk.
boot from it.
go to utilities>reset password
follow on screen information
and you're donw

Yes I did - I logged in to a different user account, and reset my own' account's password.

That's reason #361 why one should always have a spare admin account on the computer!

IS she attempting to login to the Guest or an Admin account that was setup on her machine.

If it truly is a problem you'll need the System disk to login using it.. or go to see a Genius.

Hmm, never ran into that one! That's a tough one to diagnose, I bet! How did you fix it?

I launched TextEdit and typed my account password into a black document. It then became immediately obvious what had happened. 🙂

Yeah, but how'd you do that if you couldn't get in? Did you end up having to reset the password, or log in to a different account whose password ended up not getting affected? Just curious.