For security, I use Comodo Firewall. It is honestly a bit on the technical side but I like that and it is in the end worth the extra brain time to see what is trying to connect to the internet that the Windows firewall would let through. I have found free antivirus to be enough; my top picks are: avg, avast, and bitdefender. The antivirus that comes with Comodo seems to have trouble for me, though I like it more for doing so it identified quicktime as a virus; so I usually disable it. I use AVG the most though I have been trying Bitdefender lately.
Next come System Utilities. Revo Uninstaller and CCleaner come first. For defragmentation I like MyDefrag or Defraggler. I use a combination of Speedfan - which I set to show my CPU temp in the taskbar - and Process Hacker - which is essentially a suped up Task Manager. I set Process Hacker to run at startup to keep tabs on my CPU cycles. I find Launchy to be an indispensable utility. If you check out any check out this one. It seems so simple yet can be so functional - especially on a netbook where you may have trouble with the mousepad at times - it is so much easier and faster to start programs and open folders and playlists, etc, from Launchy! Also, I use Terabyte for file transfers. It seems to be much faster with a better GUI than the standard drag and drop.
SandboxIE is a great utility for opening questionable executables or if you are visiting websites that you are worried might install crap on your computer. Clonezilla is a great backup utility - essential for not losing all your hard work! I like FoxIt reader - it works well, is small, and integrates nicely with Firefox - Oh yeah - Firefox! I love it and use it exclusively. The add-ons I cannot live without, from betterGmail to ScribeFire - which I am using to write this blog. For pictures I like FastStone Image Viewer. It is a great editor as well. Picasa works well too.
As far as multimedia software goes I use VLC for movies and sometimes music, and Foobar2000 for music. I create playlists for Foobar and store them in a separate folder that I catalog with Launchy so that I can start them with a few keystrokes.
For office software I have Abiword and the open source suite Open Office. I am also using a diary program called Efficient Diary which I enjoy and I recently downloaded a program called StoryBook for when I get to writing my novels.
Please note: Every program I have mentioned is FREE for personal use. Sorry for not hyperlinking but there are just too many to mention. I am always trying new stuff, especially new utilities, so if you have any suggestions let me know!