If you are talking about the login to Windows password then here is one way to do it:
Create a bootable USB stick using this tool: http://www.pendrivelinux.com/yumi-multiboot-usb-creator/
Use the image from Ophcrack to load onto the USB: http://ophcrack.sourceforge.net/download.php
Choose the XP LIVE CD Option
1. Download the YUMI exe to your desktop
2.. Save the LIVE CD of Ophcrack to your desktop.
3. Plug in the usb stick you want to use (make sure it is at least 500MB. 1GB if possible)
4. Run the YUMI EXE and follow the instructions. Make sure to choose the USB Drive Letter ("D:" E:" etc)
5. Choose the Ophcrack ISO for the image.
6. Install to USB
7. Click Finished and exit the program and close any other open programs.
8. Restart the computer. Leave USB plugged in.
9. As soon as the computer restarts, tap the F10 button to enter the BIOS Menu.
10. Scroll to the Boot / Advanced Menu and change the boot order to USB first, HDD second.
11. Save and exit BIOS. Computer will restart.
12. Once booted up the computer will bypass the HDD and boot into the USB
13. elect Ophcrack from the list and hit Enter.
14. Follow the on-screen instructions to have Ophcrack force the password.
(If you're using Linux then use the Linux LIVE CD Image instead of XP)