The sam file holds the pasword hashes. you can run a bootable linux distro such as the auditor CD and use samdump to extract the hashes from the sam file. you'll then need to use a tool such as john the ripper to crack them. If your system stores lanman hashes then cracking wont take too long, however if not then it could take a while and i believe you have to do something with the system file which probably requires another tool too but i cant remember what that was. You wont be able to access the SAM file if you boot into windows normally which is why the bootable linux CD is required.
Have you tried booting into safe mode and logging in as administrator? you'll probably find that there is no password and from there you'll be able to reset the forgotten password - alternatively you can probably reset the password from any administrator account on the computer. - a lot easeir than pratting about with the SAM file