Phyzik already took some programs I was going to mention: Malwarebytes, Convert and Hijack this. However I am not sure if superantispyware is that reliable anymore, I personally found that its detection rate was lower than malwarebytes. Then again different programs have different depths
For antivirus
Antivir -
http://www.free-av.com/ (It has never let me down)
Firefox addons
Noscript -
http://noscript.net/ (Great for blocking out unwanted javascripts, popups and seeing the page only you want to see)
System Stability
Glary Registry Repair -
http://www.glarysoft.com/rr.html (Great for registry cleaning but as always with registry review the entries carefully and backup first)
Media
Foxit Reader -
http://www.foxitsoftware.com/pdf/reader/reader3.php (Great alternative for reading pdf files; lightweight, light on resources, image quality can be argued but see it for yourself)
Sumatra -
http://blog.kowalczyk.info/software/...pdf/index.html (another alternative for reading pdf files;you can compare both foxit and sumatra to see which one is more desirable for you in terms of tools, image quality and loading time. Personally I prefer foxit right now)
Klite codec pack -
http://codecpackguide.com/klmcodec.htm (someone above me already mentioned it. Great for codecs and converters)
Utilities
Revouninstaller -
http://www.revouninstaller.com/ (Great for uninstalling programs and cleaning up the files and registry afterwards. As with any program, review the files and registry it is removing. Reliable, lightweight and portable)