This is not a how to hack guide, this is legit knowledge. I have had to learn this by being giving a free Unix box once, but without knowing what roots password was. I also have used this for fixing a machine that lost the passwd file due to a binary upgrade gone wrong. You must be at console for this work.
Boot to single user mode ( you know, that boot -s thing ).
Hit RETURN to get to sh
type: mount /
type: mount /usr
This will put you in that handy dandy vipw program. here you can modify the passwd files for the system. If you need to remove the root password you just need to remove anything from the data field on the line for root. Such as:
root:THiSiSTHeeNCRyPTeDPaSSWoRDFieLD:blah blah blah
you want to remove the data between the first and second colons.
Exit out of vipw and save your changes.
Hit ctrl-d to finish booting to multi-user mode.