I think that if you have followed up the advice in the first answer, you will realise that it is pretty simple to find out someone's login names/passwords and all sorts of other information IF they are physically sitting at your computer and log in as you (which in many cases will happen if they turn on your computer).
The answer to your question may depend on the operating system. For instance, Windows 9x/ME password files are stored in *.pwl files, where * is the name(s) that your Windows logon name. Although the passwords are not readable if you open a *.pwl file, they are not really encrypted in any way that can't be easily cracked.
WinXP naturally uses a more sophisticated method and the first response related to that. Although they are in the Registry, they are encrypted and/or hidden so that the average Registry Editor won't see them. (Run regedit if you know the dangers of misediting the Windows Registry and look at HKEY_CURRENT_USER\Software\Microsoft\Protected Storage System Provider and you won't see much - if you do, you need to do some searching in the Microsoft Knowledgebase!!!)
So if your question has been answered, please let us know. If not, please let us know your operating system(s), how many users you have etc. One thing that might help is for you to let us have some idea why you ask. If for idle curiosity, the answer is already given. If you want to know if your wife/husband/child/roommate can find out your password, it is a whole different ballgame.
With regard to the latter, remember that not only can someone logged in as you at your PC get your username/password combinations, but they can see your search strings and so on - unless you have all options that might save those switched off (some of them in registry keys). If you are looking at a simple means of improving security if others can use your computer, you might want to download the TweakUI for your computer. It comes under the heading of "PowerToys for Windows". It was written by the Microsoft development team for their own use as a simple means of tweaking registry keys so Microsoft warn that they don't support it (meaning no updates etc). But it works (get the one for YOUR version of Windows!!!!). One of the settings in versions before WinXP included a paranoia setting which would dump ALL that stuff when you logged out/shutdown.
Hope this helps.