Yeah, log on as root and change that user's password to the correct one. I'm assuming here that when you say "I can't log back into my computer" you mean a proper user account and not the root account.
Just boot up the machine, when it gets to the gdm or kdm login prompt, press this three key combo to get to a virtual console (a text login prompt, there are many so I'm just picking console 2 at random)
Ctrl + Alt + F2
then log in as root and run:
passwd <username here>
for example if the user was "joe" do:
and fix the password
Of course you better have the root password