NEVER EVER delete the SAM folder like one poster has advised you to. This will most likely kill Windows and require a reformat to make the computer work.
Okay, first of all all the posts that are saying to use the net user command through cmd are REALLY stupid, because this is the same thing that's available through the normal control panel to remove other users' passwords as well as having absolutely nothing to do with the original question.
To gain access to a computer or the ability to install programs, etc., the most common or easiest thing to do is log into the hidden administrator account. This can be done by typing Administrator as your log on name, or press CTRL+ALT+DEL twice to bring up the prompt if you're on the normal log-in screen that lists users. Type nothing in the password box. Many businesses don't bother to password this or don't know of it's existence. Once you log one, you can use this as "your" desktop, you can create a new account (which will eventually be noticed and deleted), or you can go into the control panel to change the main user from "Restricted" to "Administrator". However, some functions that are disabled on the regular user's account (even after promoting) are often enabled on the administrator account. Any needed options that remain disabled, to the best of my current knowledge, are done through a CP item called Group Policy, so if there is a specific thing you need access to, you can Google with Group Policy.
On systems that use log-ins with timers, such as libraries, logging into the Administrator account is usually enough to disable the time limit. The log-in is (usually) just an overlay to Windows, so if you disable the program in msconfig, it will not run and you can use the computer normally. In this case you just have to use the library's default log-in system once, then "switch user" or otherwise log off to the admin account.
If DeepFreeze is not enable (this is the software that restores a previous setup, thereby deleting any programs and files, every time you restart the computer), you can also set the computer to automatically log onto the Administrator account (bypass logon screen) or an account you have set up for yourself, by going to cmd and typing in "control userpasswords2". Just don't forget to change that option back to a different account before you leave the computer, if you don't want other eople using it or if you are in a place where employees will notice the logon has been changed and/or frequent users will report it because they prefer to not be able to use the computer themselves.
If the Administrator account has a password, you will need to remove it with a boot disk
. This site has an .iso file, which you download and then burn to a CD (you need an ISO burner specifically, not a regular CD burner - you are not writing files exactly). There is also a floppy disk for computer that have their CD drives disabled, and some USB files. The floppy option uses three disks; you insert the first floppy that does not say "drivers", then switch to "drivers 1" and "drivers 2" when prompted. The USB files can be included on any USB drive, it does not need to be one with no other files on it. Most places do not password the BIOS so this method is pretty good as far as "how many different places/computers it will work on". I have not been in any place where the USB option worked, however.
Insert the CD or floppy, then restart the computer and press F2, F11, F12, Esc, and then the rest of the 'F' keys if none of those work. The first screen you see should tell you which button to press for "boot options", but if not the ones I mentioned are the most common. It is always either a function key or Esc. Usually you will be able to access a screen that just says, "which device to you want to boot from" (choose CD or floppy, wherever you have you disc/k), but if not, you can also change "boot order" in the set-up and put the right drive first on the list, and the hard drive second. If you do this you will need to remove your disk/cs before you restart a second time.
Now you will see a DOS screen with a lot of text. Sometimes the account you want is not "Administrator", so if you see any names you don't recognize you may as well just clear all of the passwords, unless there is an account where this may be noticed. The users SUPPORT_05785789 (random numbers) and HelpAssistant can be ignored; they were not installed by the computer owners or users. There is also a "promote user" option; this makes a non-administrator account an administrator; if you think you may fail or if the Admin account breaks internet, this optin can also be useful.
After following the instructions restart the computer again. Be sure to read ALL the messages before restarting the computer, because the keys you need to press are not good examples of what keys should do, and at one point you have to tell it to "write" to save all your registry edits.
Restart the computer when it tells you to by pressing CTRL+ALT+DEL, remove your disks and log into the Administrator or other account with no password. Don't worry if the computer has DeepFreeze installed, it doesn't matter. The computer will be "reset" when you restart, but the Administrator account password will not be changed back to the original.
Sometimes you can stop DeepFreeze from working by uninstalling it with REVO Uninstaller
(get the "professional" version, which does not expire it just says it does). This only works on some computers.
Logging in with the Administrator account at most places will also stop the timers, including the automatic shut-down timer, in most places, so be sure you start saving and log off when the place is close to closing so they don't know that you got around the timer.
Besides making your own account or using the Administrator account as "your desktop", you could also use the regular desktop and set your program shortcuts to "hidden" then change the CP option to not show hidden files. (Note: After you change the CP option, you will still be able to see your hidden files on the desktop until the next time you log off and on to that account).
You should also go to Start > Run > Services.msc and disable any services that might be used to "watch" your computer from another computer. This is usually done by using another program,and those programs (VNC is one) can be uninstalled with REVO. However, sometimes that same program is also used to enable the internet connection, and uninstalling it will break the internet. You should not do anything with Services.msc unless you know what they mean. There are guides around the internet that are easily Googleable. Never disable any internet-enabling services, such as "Wireless Zero Configuration", because occasionally if the internet breaks, restarting the service will not fix it. You should, however, disable services like "WMI Performance Adapter" which use Windows to see info about what's being done on your current computer. However, using programs is far more common then using Windows services.