I just installed Windows 7/64 (dual boot) on an older system. Then I set up autologin...incorrectly.
If I boot into the the old Windows 7, I can access the file system of the new install (I think).
Is there, then, a way to find out the password?
No, the passwords in Windows are sent throught a one-way encryption and then stored in the encrypted form, so that they can not be read-out by an attacker.
You can, however, reset your password to blank with a software like "Offline NT Passwort & Registry Editor", which is part of the "Ultimate Boot CD", that provides a great choice of system recovery tools.
Warning: These tools require a lot of caution, since you are meddling with the windows internals.
Try Ophcrack (it's pretty much self-explanatory, download the iso and boot from it).
If you're not able to retrieve your password with ophcrack, you still can use this flaw in windows: How to secure Windows 8 against utilman.exe exploit
Also, if you're able to access the files from the other windows-setup, i'd recommend backing up these data first.
Several people answered your question already. There's several ways to recover a lost windows password:
Using ophcrack live cd. You download it, burn it and boot your pc with it. After while (sometimes long) it will return a list of users and their passwords. On windows7 it doesn't always crack all the passwords.
The plus side is you don't need to change your password, you actually find out what it is you forgot.
Another option is to use one of the several windows recovery tools available:
Another option I think no one mentioned, is a quick hack you can do from any linux live cd or windows recovery console:
konboot - that'll let you login without a password
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