I have not used AV software in almost 3 years. I check with Housecall or my USB drive every couple months to be sure I am safe though. I just dont want that footprint on my system. If you want to use one that scans everything all the time etc, get AVG Free. Also I would suggest Ad-Aware.
If you need to use PDF's I would strongly suggest Foxit, it has super fast load times, but you will still need the Adobe Firefox extension to view them in Firefox.
Chances are you will need Java Virtual Machine, but most people don't really use Java programs outside their browser, which will prompt you to install the plugin when it is needed. If you know you will need it independently of your browser at some point, now is a good time to download the JVM.
GIMP is a great (and free) Paintshop alternative if you have occasional use requirements.
If you have any other software needs, or just want to browse what is available as for open source alternatives to major software titles, check out:
Software | WHDb