I'm going to post a link to my must have software in terms of convenience and performance. Everyone feel free to add to the list. Also this isn't in any particular order, just the order they are in on my HD.
AIM Ad Hack -
http://aftermath.dyndns.org:8888/~aimadhack/ removes all adds and spyware related stuff from a fresh AIM install. There is a version for AIM triton and AIM 5.9, just look around
Mediaportal -
http://www.team-mediaportal.com/ for those of you who have a TV tuner card you cannot beat this free software. My gaming PC duals as my HD theatre due to this software
Ad-Aware -
http://www.lavasoftusa.com/software/adaware/ Spyware removal....free and delightful
Azureus -
http://azureus.sourceforge.net/ my bittorrent client of choice (+safepeer plugin)
MS bootvis -
http://www.softpedia.com/get/Tweak/System-Tweak/BootVis.shtml MS program that improves boot time. i love this thing
Crap Cleaner -
http://www.ccleaner.com/ remove all the crap from your computer in one click, also fixes registry problems
Driver Cleaner -
http://www.drivercleaner.net/ If you are like me and install new drivers every second they come out then use this, it will save you lots of time
Firefox -
http://www.mozilla.com/firefox/ you better know about this by now
Google earth -
http://earth.google.com/ free and fun, find directions, etc
Image resizer powertoys -
http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx simple image resizer given for free by MS...yay!
MS antispyware -
http://www.microsoft.com/athome/security/spyware/software/default.mspx Free (for now) antispyware. use in tandem with adaware
TuneXP -
http://www.driverheaven.net/dforce/showdoc.php?doc=txp_download awesome tweaking program to get every last bit out of your computer
That is the main ones i can think of for now. of course there is winzip and winrar, some CD burning software, and antivirus software. Add on yall!