Windows 2000 introduced a new concept called "slipstreaming".
Slipstreaming means once you install a service pack, you don't need to reinstall it even if you install new applications (either Windows or third party apps). It's best to install theservice pack before you install anything else, but it's not required.
Threee other tips:
(1) Service Pack 3 has been released for Win2k. (2) Never (ever) install Win2k (or any other OS) while it's hooked up to the network.
(3) Don't use a newly installed system to download service packs. Download the service pack from a machine that is already patched, put the SP on CD, and install it to the offline machine. I've personally witnessed Win2k computers getting infected with Code Red, Nimda, and other "bugs" *while* the SA was downloading the service pack.