Check out the following articles for how to use a Linux tool called chntpw (which being translated means: change NT password) to delete a Windows password.

I have used a live-cd that I found here to delete passwords before.  Basically, it boots up a minimal Linux instance and uses the chntpw to delete the passwords.  This tool has the added benefit of automatically finding the location of the SAM file.  The guides above, however, have the benefit of working from any live-cd that has an internet connection (so you can use apt-get to install chntpw).