Which OS are you running? Can you boot into safe mode? I know in Windows XP there is a guest account that shows up in safe mode. If you could use that you can use the command prompt to run this command
net users (your user name goes here) "password"
Whatever you put in the quotes will be the new password. So for example:
net users Sally "newpass"
That would change Sally's password to newpass. However we will still need more info on your current system so we can make sure you have the correct answer. Also one more thing....this master password you are talking about. Is this for an account in windows? Or for the cmos? If it's for the cmos the above recommendation won't work.