|Lotuscather's info applies only if it's just Windows you don't know the password for. |
You can also re-install Windows from scratch.
If it's a system access password - you must type it to use the computer at all - or a hard drive access password - you may be able get into the bios Setup but you must type it before the operating system will load, then.....
Removing passwords on modern laptops is difficult or impossible, unless you are the original owner and can prove it, or unless you are willing to spend money to have an expert remove the passwords, or you are willing to spend money to replace the mboard, and the hard drive if the hard drive is passworded.
For more info see response 3 in this:
There are illegitimate ways of removing laptop access passwords, but the rules of this web site forbid us from telling you about those.