thats pretty easy, i'll quote [http://manuals.info.apple.com/en_US/Snow_Leopard_Installation_Instructions.pdf|apple] on that matter:
"'''You don’t remember your password and need to reset it'''
If you don’t remember your password and you have the Mac OS X installation disc, insert the disc and restart your computer while holding down the C key. If your computer doesn’t have an optical drive, use Remote Install Mac OS X. When the Mac OS X Installer appears, select your language, choose Utilities > Reset Password, and then follow the onscreen instructions."
very short version, boot from your setup dvd, go to the utilities, reset the password and quit the setup