First and foremost I am only stating my opinion about these programs and have no intention to construe it as professional technical advice. I'm just saying that these programs are ones that I found to work quite well on my PC for my specific needs and they are free. I go back far enough to still have the spirit of open-source and freeware. Free things are cool.
I often hear people ask why would a company would give things away. The same reason that lady stands just in the greeting area of the super-market with the meat on a stick all laid out on a table. "Here try this". A fair sales practice in my opinion.
The recent overwhelming infestation of ad-ware, mal-ware and all the other vermin variants out there pillaging client's machines has caused me to add the IOpian Corollary to Free Stuff.... Nothing is really free... Everything has a cost... like a conservation of energy thing.
Most Importantly Remember...
Beware of Geeks Bearing Grifts.
There was so much of this that I started to see a pattern. Usually the person had an isnstant messenger running either Yahoo or AOL but it was Yahoo IM that was installed.
So I infected myself to see how this was happening. There was a page that offered about 7 or 8 'browser experience enhancements', free animated icons, web offers and that kind of stuff. They were checked by default and within a few minutes my start page was hijacked. I started to see trojans spawn processes. Delete them and there they are with a different name. There were many and I won't go into detail.
It does demonstrate my observation that users will not read what's on the screen.
Now to be fair I've seen many machines running Yahoo IM with no such problems. I won't even discuss AIM because I'll just go into an entire rant on them in general starting with Version 5.0. Another time perhaps.
Another common thing I would see were custom screen savers like the one that updates daily, I don't remember. Can't say this caused any problem, maybe a coincidence.
Then there are the web pages with lots of things jumping around to get you're attention like smiley icons and mini movies ...'hey click me and let me infect you...'. Could just be my paranoia but I stay away from them. While we're on it things moving around... why would you do business with somebody that pops a banner up in your face ? I will never do business with Orbitz.
Anyway all these barbarians beating at the gate caused me to fortify my defenses. Since my defense budget is in the red I had to do so by the cheapest means possible. Also I am....well cheap.
These items I list are just the programs I have found work for me on my system.
Antivirus Protection -
AVG Free Edition 
The problem with most retail packages is the concept of a subscription. You pay for the AntiVirus program initially and get to use it with free updates for 6 months. Then you are nagged to renew the virus profile which is essential, absolutely necessary, so you are paying for this thing every 6 months or a year. You are actually renting it.
Here's the problem I have with that. They are only selling you a license to use their property - the AV engine. So the program is theirs. What have I paid for? Protection from viruses. Period. Don't care how you do it, I've paid you your money now do what you said. I want to pay for it once, register it properly and it be kept updated by the owner.
I searched and found the AVG Free Edition by Grisoft. I put it on all my computers. This edition is not full featured but it does update itself daily and has not caused any problems whatsoever.
This product is not fully featured either but it does provide firewalling. It lets you see when the critters try to phone home to tell their friends there's a party on your PC. Then it catches them coming in. It has to be trained as to what is allowable traffic and what is not. A nuisance at first because it starts out dumber than stone. Eventually it massages out and pops up only when you really need it to pop up. Good stuff.
Online Virus Scan - HouseCall by Trend Micro
If your machine is dogging down, things are popping up all over the place the first thing to do is go to Trend Micro's online virus scanner HouseCall. It's going to ask to install a program. Tell it yes and let it rip. First time could take a while. If it finds a virus it will attempt to clean it. Good idea to write down the trojan variants it finds in the results. Then you can google those variants and sometimes find the executible. Sometimes not.
Popup Blocking - Google Toolbar
There are many of these toolbars. Some are loaded with spyware and ad-ware. The google toolbar is a safe choice. I like having the Google Toolbar on my browser wherre I can just copy and paste words and phrases into the search. Just the way I work. But it will block most popups.
Blogger Bot and Picture Sharing Utility. - Hello
This is a very cool little tool for your Blogger.com account. In the past I would ftp my graphics to one of my other internet accounts. Then I would have to go back to blogger and insert the graphic path into my code. This isn't that big of a deal but Hello is integrated so that you select a picture and send it directly to your blog. Then you can copy the code from that post and insert it into anoter post. Sort of a staging area for graphics and free image hosting. Really nice. Caused me to want to start my blog up again because now it takes less effort.
You can access Hello from the " more >>" selection on the Google main page.
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