Mad Boy, after the operating system, I think every pc has to have an office suite, then to me the most important is a graphics package. I have CorelDraw 7. It cost an arm and a leg back in 1996, it is full of bugs, but I still find it indispensable.
After the o/s, word processor, database, spreadsheet, (Don't need to be top range)dictionary, knowledge base program of some sort, Route finder, DTP, (Publisher or Serif), anti virus, PS Pro, plus any printer/scanner/CD software you require.
This got me thinking and it occurred to me that, after all the obvious things such as Operating System, Browser, Email client, Anti-virus/Firewall/Security, WP, DTP etc., the software I use most -- and find most useful -- is my Password Manager, AnyPassword ( click here ).