if I were the previous owner I would have formated the hdd before I gave you the system. The would be to insure that you didn't get any personal data.
its not additional security. More than likely he has set it up to require logging in with the proper password. You said this was Windows 2000, right? It should always come up with ctrl/alt/del to enter id and password. If its not then the os might be corrupted.
if this is an hp or dell it should have come with a manufacture restoration/os cd. If you don't have one then you will need to take it to a local shop. I know several ways to get into the os, but they require a fair amount of technical savy. Not to mention that if you can't bring up the login screen there isn't much one can do.
.....on second thought if you could be more specific as to what it boots up to that would help. also since you started a second post on this instead of continuing with the original it would be nice if you provided a link to the original post. WOuld make it much easier for those us trying to help you.
Starjax