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Possible Duplicate:
What can I do if I forgot my Windows password?
I forgot the Windows 7 password on my new computer and I can't log into it at all. I was the administrator for the computer with that account.
Now I have to use my old Windows XP computer to ask for help.
How can I change my password to something else so that I can log back into my administrator account?
When doing this for friends I typically use a Ubuntu live CD and the install and use chntpw.
Boot from Ubuntu. Go to System > Administration > Software Sources, and enable everything. Use one of the various methods to install the software (depending on which version you have). I just typically prefer to open a terminal (Applications > Accessories) and type
sudo apt-get install chntpw
You should then be able to browse to the hard disk which will mount it automatically. Getting to the password file, and using the tool is pretty straightforward, but you should look over some of the blogs and instructions available, something like http://www.howtogeek.com/howto/14369/change-or-reset-windows-password-from-a-ubuntu-live-cd/ to get a feel for what is going on.
I just did this a couple of weeks ago on Windows 7 using Ubuntu 9.10. The biggest thing for a non-Linux user will be getting to the hard disk from the command prompt. If I remember right, after I browsed to the disk, I opened up Places > Computer, then right clicked on the mounted drive and looked at the properties to get the path. It will be different for every situation so listing it wouldn't be much help, but it wasn't very hard to figure out.
There is also apparently a live CD for just this purpose which I have never used, but basically takes the same approach. See http://pogostick.net/~pnh/ntpasswd/bootdisk.html.
Ophcrack is to use in malicious case: to stealth a password, and if you have a strong password Ophcrack could take many times to find it...
You should rather use Offline NT Password & Registry Editor (tutorial) to reset your password.
Update:
Note 1: If you have some EFS encrypted files, reseting the password doesn't give you access to these files, so you should use Ophcrack.
Note 2: To boot Offline NT Password & Registry Editor from a usb drive, two choice:
Using the Bootable Floppy Image and unetbootin: launch unetbootin > Select ("Disk Image" > "Floppy" > "...") the Offline NT Password & Registry Editor .bin file (contained in the zip) > Select your USB drive > Click OK
Using the Bootable CD Image, an iso extracting program and a command line
f:\syslinux.exe -ma f: (replace f by the letter of your usb drive, beware of not using another drive letter, it could render your system unbootable. If you get an error, you may have to omit the -ma option. If it says nothing, it probably did install the bootloader.) 
My favorite tool for resetting Windows passwords is the Trinity Rescue Kit Linux live CD. All you have to do is burn the ISO, boot from the disc and use the included scripts to change the Administrator password.
Once TRK has booted, run the mountallfs filesystem mounting script to mount local drives:
# mountallfs -g
The -g tells the script to mount NTFS drives in read-write mode. Once mounted, run the winpass password reset script:
# winpass
...and follow the prompts. Reboot (shutdown -r now; don't forget to take out the CD) and login with your new password.
As mentioned by @fluxtendu, you will lose access to any EFS encrypted files. Additionally, you're pretty much hosed if your Windows installation is on a BitLocker-protected drive.
TRK is also useful for a host of other recovery scenarios; it's saved the data of myself and friends from unbootable computers several times.
I am not sure whether it works on Windows 7. It works on Windows XP and Windows Vista without any problem (though it was developed to crack Windows XP, it works on Windows Vista too).
There is one program, namely "OPHCRACK". You need to take help from anybody. To get it written on a CD/DVD (make it bootable one).
Insert the CD and restart the system. Allow the CD to boot and crack the password.
