What operating system is your laptop running?
On Windows (NT/2000/XP, not sure about Vista) remove the c:\windows\system32\config\sam file to delete the password database. This resets the admin password to '' (blank). Probably safer to move the file rather than delete.
Normally this file is protected. You won't be able to access it as guest.
You could boot from a floppy disk to delete the file using the command prompt. If the SAM file is on an NTFS partition, you will need a special boot disk to access the file.
Alternatively, remove the hard drive, and attach it to another machine using a USB caddy, and delete the file from there.
Or get a Windows CD, install Windows to a new location, boot into the new Windows install, delete the SAM file from the old location, reboot into the old install, and delete the new install.