Can't get access to my user account on my Laptop

I have a password on my computer and the only way to change it I believe is to go into user accounts on the control panel but it won't let me do this.Everytime I right, left or double click on the user accounts icon nothing happens.
Anyone know how to get around this as I'm looking to sell my laptop, a Compaq Presario V5000 but it can't be refurbished unless you have access to the user/admin account.I'm the only one who has used the computer and I can't figure out how I've locked myself out

Thanks in advance

No Lipps

I don't have the disk

I'm not too bothered about losing the data that's left on it as I have it downloaded i to USB sticks I just need to get in and change the password settings and have it wiped clean