Password won't work

A little while ago I downloaded the beta version of Lion which turned out to be total crap and slowed down my system, wouldn't let iTunes work, etc. So earlier today I downloaded 10.6 again and installed it onto an external drive to run the system on. I restarted the computer, held down the 'option' button, installed 10.6 completely, but now the password won't work on either of the accounts on the computer. I know I'm not getting the password wrong because in the password hint one of the accounts I literally put what the password is. Is there any way that the computer could be getting the password wrong, if so how can I fix it?

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