You can start your computer in safe mode and get passed the sign in then delete the account who lost there password... but it depends who has administrative permission.
restart your computer push F8 until it askes you how you want to start windows
start up in safe mode
should give you the option to use administration account or parents account... click admin
once the desktop loads
Start/settings/control panel/user accounts
then you can delete the old account and start a new one
or you could just open a new account so you can use the computer
which ever way you choose make sure you give your self administrative options..
( i know you can get to the user accounts using the above options but depending on administation options you may not be able to delete the old account)
good luck