Hi,
If you're running Windows 8 or 8.1, see the Microsoft page on the following link.
http://windows.microsoft.com/en-gb/windows-8/forgot-windows-password
Note: If you have a Local Account, try the free application Here.
If you're running Windows 7, Vista etc, try the following.
Using another PC, download the application on the following link and save it to your Downloads folder.
When the download has completed, open your Downloads folder and double-click the installer - this will result in another file appearing which is the disc image ( .ISO ) that you need to burn to a blank CD.
You will now need to use an application such as ImgBurn to burn this ISO correctly to a blank CD - a guide on using ImgBurn to write an ISO to a disc is Here.
When the disc has been created, shutdown the PC you can't log-in on, tap away at the esc key as soon as you press the power button to access the Start-up Menu, insert the CD, select Boot Options ( usually f9 ), use the arrow keys to select the CD/DVD drive and hit enter - you may also have to 'Press any key' to boot from the CD if asked.
Follow the instructions on the guide Here to remove the current password.
Regards,
DP-K