Here is a list of what I consider to be "must have" software, all of which are free.
1.
AVG Free (AntiVirus)
2.
VLC (all purpose multimedia player, plays everything)
3.
WinRAR (Decrompession/Compression software, .zip, rar etc)
4.
Firefox (obvious)
5.
TVersity (UpNP media server, serve media to xbox 360, PS3 etc)
6.
RivaTuner (Video Card tweaking tool, overclocking, fan control)
7.
Rocket Dock (Create as OSX'ish dock, custom apps etc)
8.
CCleaner (System cleaner: temp files, registry etc)
9.
Handbrake (Transode, encode your video files to .mp4, h.264 etc)
10.
Malwarebytes (Spyware/Malware protection and removal)