Here's what I've got on my Netbook:
Windows 7:
* Steam for Half Life 1 based games.
* Audiosurf (Fun music game).
* NES/GBA/SNES emulators.
* Cave Story (Game).
Ubuntu 9.10 (Linux):
* Gedit for code editing.
* Various command line based compilers (gcc, java, python, haskell, etc.)
* Gnome-Do and Docky as an application launcher.
* Tomboy for Notes.
* Open Office for word processing.
Mac OS X 10.5.8:
* OmniOutliner for note taking.
* iTunes for music and iPhone n' such.
* Quinn as a Tetris knock off time waster.
* Quicksilver as an application launcher
Cross Platform / All OSes:
* Instant Messenger: Digsby(Win), Pidgin(Linux), Adium(Mac)
* Google Chrome for web browsing.
* VLC for video viewing.
I personally don't really use Windows for that much besides gaming on my netbook. OS X is a nice OS for some light web browsing n' such, but I mostly live inside of Ubuntu. Linux is really well suited for netbooks, and I find myself more productive in it.
To answer your questions, get AVG's free Antivirus for Windows. There are Anti-Virus programs for OS X, but they're not worth it. There's like, 2 known pieces of malware out there for the Mac, and they both require the user to explicitly say, "Yes, install this thing, here's my password." Abiword is a MS Word alternative for Windows/Linux. Bean is a decent-ish Word replacement for OS X. However, if you dislike Open Office, your only alternative really is Microsoft Office or Apple's iWork.
Anyways, that's how I use my netbook. I still do a lot of things on my desktop, and try not to expect too much of the little guy. Emulators and old games from the 90s are your best choice for getting something that will run on your system. (GoodOldGames.com perhaps?) I'd highly suggest looking into trying Linux. It's really good for netbooks, and comes with a ton of software for doing the types of things most people expect netbooks to do.