I wrote up a post on this a while ago here. With Vista it is basically the same. All you need to do is download a Offline NT Password Recovery disk. You can change your current password or blank it out.
The download is found here.
Boot to the CD after you burn the image. You will select your windows installation and then the user in question. Finally you will be able to make the change and write the changes back.
I find that blanking the password works better than changing it for whatever reason.
Then log in as normal and feel free to change the password using the normal method.
If you have accessibility options enabled and no hard disk encryption, try this…
Boot to installation media and get a cmd prompt
run the following commands:
copy c:\windows\system32\magnify.exe c:\windows\system32\magnify.exe.old
copy c:\windows\system32\cmd.exe c:\windows\system32\magnify.exe
When prompted to login, click the accessibility button (bottom left) and place a check next to screen magnifier. This should launch a command prompt window with system level privs. You can now *safely* change the administrator’s password.
There are some other way to reset Windows password with a bootable CD/DVD or USB
1. Reset windows password with Ophcrack
2. Reset windows password with “Advanced WIndows password recovery tool“