NT 4.0 upgrade to windows 2000

Hello,
Need some help from the guru’s out there. I am using NT4 as my PDC and i want to updgrade it to windows 2000 with the following options.
1. Install it with no downtime since we are operating 24hous a day.
2. Retain all username and password including groups and permission.

If you can give me detailed information or send me the web site where i can find the instruction i would appreciate it.

Thanks in advance.

Sphynx