Is this a BIOS or Windows password? That's the first question. If it's a BIOS password you'll need to crack it. Quite frankly you're on your own with this. I'm not helping you crack the BIOS passwords.
Windows passwords are easy to fix. You just need to find a utility that will work. But for Win8.x, you need something that doesn't mess with the secure boot. If you use the wrong tool you're rewriting the boot keys.
For Win7 and older, most tools will probably work. I like Kon-Boot, but there's other tools out there.
You can also use Command prompt too. Read below on this.
Press F8 at startup, and select "Safe Mode with Command Prompt" and let it boot into Command prompt. Use your install DVD if you have problems at all with the existing install.
When in command prompt, enter netuser in the command
Find your username and enter netuser username password
Put the account in question in place of username
Pick the new password.