Below are links to some of my favorite free tools. Be sure to check back often as I am always evaluating more tools.
Microsoft Security Essentials
Microsoft came out with their free version of antivirus software. It is good for the average computer use who is doing basic office and internet surfing. Updates are a breeze and run in the background for my customers while I'm working on other things. On top of all that it doesn't use up very much memory. If you want something more robust, try Eset Nod32 by clicking here. It will give you a free 30 day trial. Please uninstall any other antivirus before installing either Microsoft Security Essentials or Eset Nod32.
Superantispyware is a great program that will catch things that your antivirus will sometimes miss. Be sure to update it before you run it. It's a manually run program. I like to run it 2-3 times a month to keep my computers in good shape. If you like the free version, the paid version offers slightly better benefits. It is safe to run in conjunction with any antivirus package and works well with Malwarebytes loaded on the same computer.
I have used a lot of different anti-spyware packgages over the years and I'll have to say that Malwarebytes is by far my #1 favorite right now. It has a lot of great features and is an on demand scan. If you have any Antivirus package protecting you then there shouldn't be a need to have an antispyware application running too. This is where Malwarebytes wins big with me. Download it and check it out for yourself. I typically only use the Full Scan feature on the first tab and the Update Tab when necessary. Be careful with the other tabs. You've been warned.
Trend Micro Clean Up Tools
These are great tools for your computer and phone that do quite a bit and it's hard to believe Trend Micro gives them away. The main tool that I want to highlight is House Call for your computer. It is a downloadable application that will install and run and helps to get rid of viruses and spyware that your normal antivirus and antispyware can't get rid of. Everything else here is use at your own risk.
This is my favorite tool above all else. I've used it for years on my own computers to keep them in tip-top shape and it has done a great job with my customer's computers too. Download it for yourself and check out some of the other tools from it's author, Piriform