Useful, and Essential Applications - These are the programs I use on a regular basis. Mostly FREE too, so get your mouse clicking that SAVE AS button :)

... Hey look everyone, I eventually made the hyperlinks I been promising for past year or 2! ;)

Winamp What you doing still using that 'media player' junk, check out Winamp, simply the best music/video player for your computer, I've used this for, oh, probably around 7 years :) www.winamp.com
Winzip Anyone remember PKZIP/PKUNZIP? If you do, you been here a while, Basically, this is a file compression/decompression tool, many of the files you download online are already compressed using this :) www.winzip.com
Norton AV Do I trust anything else? Sure, I get people saying 'x' program is better, but then I invariably come across someone's machine using the 'x' software, and still having viruses, so... Yes, some catch more than others, Norton is a good all round virus scanner. Recent version do seem to effect computer speed! www.norton.com
AVG Free Antivirus Once upon a time I'd have not used this, I found it was very poor at detecting viruses, today however, it's better, it's changed.. I'd still say Norton was *the best*, but AVG has to be a very close second, and probably My preference :) Does not seem to effect computer speed to any noticeable extent, Recommended! www.grisoft.com
Spybot S&D Spyware has been around a good few years now, and a complete pain, see www.grc.com for info on these as that's the site that first brought it to My attention, many years ago. Latest problem is browser hijacking, every ended up at a page that you didn't expect? possibly your browser has been hijacked. Anyway, this tool will help get rid of the problem :) www.spybot.info
Nero - Burning Rom Good all round solution to backing up data/music/video, or making your own bootable discs. www.nero.com