How to Be Sure It Will Be Possible to Revert to Previous OS Boot?

Hi.

I have a computer that's running Windows 2000 only, right now.
I am quite sure it should be capable of running Vista, too.
What I'd like to do is dual-boot 2000 with Vista.

The question, now, is, if I'm unsuccessful in getting Vista to run like I would expect, then I would like to take out / format the hard-drive that had the now-unwanted Vista partition, and go back to just booting into Windows 2000. So, how can that be accomplished?
Should I just copy the current boot.ini file, or something, first, and then put it back if I decide to take out Vista?

I understand that, if I install Vista, the new boot information would be on the Vista partition, and the current Windows-2000-only boot information would be overwritten and lost. That's why I'm asking, you know, because I'm not sure what to keep, in case I have to go back to just one OS.

Thank you very much.