I have used CCleaner almost from the first day it was released - where the C meant something.
It is SO great.
I've also got AVG and ZoneAlarm. AVG has gotten top heavy though and I've tried out other applications including Avast which seems to be just fine. AVG no longer gives you a way to suspend it for more than 15 minutes. Probably a good thing for a casual user.
No firewall will ever top BlackIce - but it was never upgraded beyond Win98. So sad.
I've used Ad Aware but replaced it with Spybot.
I also like FREE Microsoft Process Explorer - but it's very technical. Gives you WAY too much detail on what's running on your computer. Fascinating.
I have MS Office, but when it's no longer supported or have to upgrade, I'm off to use Open Office. I've heard really good things about it.
I use Mozilla Firefox and I like the Add Ons that are also free. (Tools, Add Ons, Get Add ons) There's one I didn't load that saves pages for you to read later, but it looked great. One I did, though, Fireshot, is nice. Similar to SnagIt (though that's not free any more) it will capture a given area of a web page. So you can save just a picture or just text or whatever you need. You have to be careful with add ons though. Some people advertize their add ons as free, but they're really only demos and they start hounding you for money or stop working after a certain time. The ones on Firefox have reviews though, so it should be easy to find good ones. If there's anything you wish your browser would do - there's an add on for that!
If you can find any old downloads of SnagIt, you can capture any part of your display in the form of a bmp. It would scroll too, but doesn't work with tabs.
I can't think of any more right now. I like to use free stuff and then, if it's any good and I know I will use it, I'll send a few bucks to the author. I think that's really nice - for people to offer help. I suspect they end up making more money that way, too. :-)