Consider this a companion piece to OCC3 since that's been the cause of most of this prep. You're going to need a bunch of software to live on 2k, almost all of it outdated. This is going to collect some specific versions and general tips. I've linked to project sites where possible, tracking down the correct version is up to you. I also take no responsibility for whether any of the sites have been taken over by malware, etc. The good news is that if you got something I probably got it too, and 2k is so old that there probably aren't many exploits floating around targeting it even if you do get hit.
2000 is the last version of Windows without native support for zip files, so you're going to need something for that. This is easy: the latest version of 7-Zip supports 2000.
Old sources will tell you to install Opera 12. While Opera is the best for pure compatibility, it's completely unusable now because of the web's silly insistence on TLS everywhere. You'll actually need two browsers because some pages will crash one and vice versa. The first is K-Meleon, the second is Retrozilla (this link may crash k-meleon and won't work on either. You'll need to bootstrap this from another computer.)
A surprising number of GOG installers work on Windows 2000 as long as they're the correct vintage, so give them a shot. Otherwise take a look at abandonware sites.
Honestly notepad pretty well sucks. Notepad++ as far forward as 6.6.6 works fine.
I believe emacs....20 also supports 2k, but I haven't set that back up.
For pictures and similar you're looking for IrfanView, the classic. 3.97 is the magic version. For videos, DVDs, and the like there's VLC, where you'll want 1.1.11
A bit of a catch-all, but: you won't have access to ssh as it won't negotiate protocols correctly, but you can use PuTTY versions as new as 0.61. For FTP you're looking at 3.0.11.1 of FileZilla. The only working mail client I've found is OE Classic, which explicitly continues to support Windows 2000.
You'll be spending a lot of time on abandonware and old versions sites. Major Geeks and Old Version will be some of your best friends.
If you find yourself needing a website which just won't load, throw Frogfind at it and see if it'll work. If you just need some stuff to look over in general while you're bored there's always the Directory too.