This is my first instructable, so don't be too hard on me.
This instructable basically sums up most of the login bypassing methods posted on Instructables.
Step 1: The golden disclaimer
First, the standard disclaimer that goes with any good Instructable: This is to be used for instructional purposes only, and I am not liable for any damage or destruction to you or your computer that may occur from attempting this Instructable. In other words, if you delete your registry or mess with a school computer and get arrested, it's not my fault. And don't use this to get into someone else's computer either.*cough*
Step 2: The easy part
Ok, now to start.
If you have a login page like the one in the intro, go to step 2. If you have a login page like this one, it should be relatively easy to get into. Unless the default administrator account has been changed, all you have to do is type "Administrator" or "administrator" as the user name and the password will be either blank or "password". Then press "OK" and you should be in.
Step 3: The nastier part
If you have a fancy windows login page, things are a little more annoying. First, click "shut down" to shut down the computer. Then turn it back on. As soon as the screen turns on, press and hold F8. After about 10 seconds, a boot options menu will pop up.
Step 4: Safe mode
Now, in the menu, which should look like the picture below, select safe mode and press enter. It will show a whole bunch of gobbleygook for quite a while, (which you don't need to know anything about)and then it will bring you to the login page. This time though, there should be an account named "administrator". Click it, and either type "password" or nothing, whichever one works. If you don't get in after that, you're bummed.
Step 5: The guest account method
If nothing so far has worked, but there is a non-passworded guest account, you may have hope yet.
Go into the guest account, click the start menu, click run, and then type in "cmd". A black window should pop up.
Then, after typing "net user administrator *" , press enter and you will be given directions to create a new password for the administrator. After typing in the new password, press enter and close the window. Log off, then get into the administrator account with the new password.
Step 6: If you get stuck
If you try this and it says "access denied", you're out of luck. :(
(If its got security, you probably shouldn't be in it anyway.)
Well, that's the end. I hope this was useful, and if you have any questions or suggestions, please leave a comment.