login: root
password:
# passwd
Something like that should do it...
Or if you can't log in, boot a rescue cd, and mount your HDD's / partition. Edit your HDD's /etc/shadow file or (passwd if not using shadow passwords) and remove the second field of root's line, eg root::12242:0:99999:7::: (Leave everything after the colon alone, just remove the second field).
BTW, that kiddies, is why you might want to use boot manager passwords and/or encypted filesystems.
<i>Knock Knock, Neo</i>