I like CurseTheSky's list too, but a few things I diverge on:
Paint.NET or IrfanView for general image editing
Paint.NET is a good one, not a big fan of irfanview, mostly because of the authors openly condescending stance on supporting unicode filenames. I think Faststone/XNview are better programs to begin with anyway.
CCleaner for general junk cleanup / registry cleaner
I hear most of these programs don't really have much benefit, cleaning empty MRU list and slack space is about the biggest things they accomplish in most cases.
Pidgin or Trillian for instant messaging
I never liked Trillian, but it's a matter of preference I reckon. Miranda is my pick for being lighter and more customizable, but it may take some work to get some of the plugins to work.
Avira Free or Microsoft Security Essentials for anti-virus
the MS one seems to get good reviews, is it as light as avira or even lighter? Maybe I should check it out one day.
Other programs I would tack on to that list include:
foobar2000 for music playback, potplayer for video playback, revouninstaller for uninstalling apps, virtualbox to experiment with/run all the nasty lil things you don't want on your computer.