Here are a couple of must-haves on your PC (and all of them are FREE!):
Avira Anti-Virus - I've been using Avira for a couple of years now and I never had any problems with viruses. And what's great about Avira is it's free and there's an update every few days.
Open Office - I've been using Open Office for over a year now and there's really not much difference between using it and MS-Office. Most of my documents though are on Google Docs since I oftentimes change the PC I use. And it was a hard lesson learned when George (my old PC) crashed when I was out of town, good thing I always backed up my files.
Google Desktop - I love using Google Desktop because I just need to press Ctrl twice and I can easily search for a file in my desktop. I also use it for some RSS feeds, notes, a Picasa slideshow of my photos and it's where I anchor my Gtalk.
Picasa - I use Picasa to keep my photos organized. I love the fact that you can have your name/URL stamped automatically on the photo when you upload it to your web album. My other favorite button is the "I'm feeling lucky" button which edits your photo automatically. And I also use the "Blog This" button and I'm currently learning how to edit videos using Picasa too. Plus you can also use Picasa to import photos from your phone!
Gtalk Gadget - I use Gtalk 99% of the time now, so I just use the web version of YM. I like using the gadget because it's easy to send files and chat (by voice) with friends. Gtalk on Gmail is also quite exciting with the video messaging feature.
iTunes - Can't live without music. `nuff said.
So those are my standard installations whenever I have a new PC/laptop. And it's all FREE!
What about you? What do you install on your PC/laptop?