Anti-Virus: Avast or AVG (I have Avast on main PC, and AVG on laptop, mostly out of laziness, cos I had AVG's exe on the lappie already, but also because I rank them equally), and Spybot (always have Spybot around)
Firewall: I'm behind a router, so no need
Defragmenter: Vista's defragmenter is working fine, just like XP's, plus, it does its job automatically, and I've never noticed performance problems with it, when its doing its thing, so I just leave it be.
Torrent downloader: uTorrent is what I use on all PCs, and is brilliant, much lighter than Azureus, and does its job quite well. Love the WebUI
CD/DVD burning: Nero 7 Ultra, which someone gave me (they ended up getting 2 legal copies somehow), it's brilliant, works more than well enough, but failing that, Nero 6 comes with, or used to come with, most CD/DVD-RW drives, and is more than adequate. Nero also has a free Burning ROM download on its website, but I can't find it most of the time.
Web browser: Right now, I'm on Internet Explorer 7, but only until opera 9.5 is released. I even use Opera Mobile, and every system, I've used within the past two years has had Opera, either because of my U3compatible drive running it, or cos I installed it proper. Great web browser.
Photo/Image editor: Depending on the task, I have GIMP, Paint.NET, and Photoshop CS2 (students get a huge discount, if anybody didn't know)
Media player: WMP11 works just fine, but I normally have WinAMP in use. I tend to stick to older versions, which come without the bloat.
Other: Blender3D, I like to do a bit of 3D modelling occaionsally, nothing special, and I ended up getting used to Blender
ShareKMC, use a laptop, or another computer, as an extra display. Completely free, works with Vista, next to no lag, even over wireless network.