For me it varies since a good deal of time usually passes between the times I need to start over and my preferences change. However, if I were to get a new computer today this is what I'd install first:

Browser: Google Chrome (I'd install most major browsers, but whatevs)
Unpacking: 7-zip
Music player: foobar2000 (for locally stored music), Spotify (for streamed music)
Video player: GOM Player (for locally stored videos), Voddler (for streamed videos)
IM etc.: Windows Live Messenger, Digsby, KVIrc
Antivirus: AVG Free
Office Suite: OpenOffice
Torrent client: µTorrent
Other: Adobe Master Suite, Notepad++, Dropbox, Daemon Tools, Flash Player, Foxit Reader