If you're using Lion, reboot from the Recovery partition (hold down cmd-R while you restart) and using Terminal in the Utilities menu, type:
Then follow the prompts.
If you're running 10.6, reboot from your install DVD (insert the disk, hold down C to boot from it) and locate the Reset Password item in the Utilities menu.
G'day Matt, cheers mate... I've done that although at first I had a Bluetooth keyboard which half way through I figured wouldn't work so I took my finger off the c key and went and got a cable keyboard. Restarted the computer while holding down the c key with install disc in and all I get is a white screen with a grey apple insignia in the middle, I held my finger on the c key for about 10 mins!? Any help with this will be greatly appreciated, I'm hoping I haven't had a mare with this!?:/