Keeping your computer malware/virus free is sort of an arcane art. For example if you install everything on this list and have it automatically run at startup your computer will be slower due to everything loading into memory. Many of these programs have a 'portable' or 'standalone' version which means you don’t have to install them to use them.
Here is my recommended configuration:
(loads into memory at startup)
Microsoft Security Essentials
(Only run when you tell them to)
My Favorite Free Anti virus is Microsoft Security Essentials
It is free and light on system use. It also has important features such as automatic updates, scheduled scanning and real-time protection. You'd have to pay for these features in other programs.
For programs I like to use when cleaning the registry or scanning for spyware:
My weapon of choice against spyware
Bob's weapon of choice against spyware
Also of note: Spybot Search&Destroy. It has a feature that allows you to block sites known to be infected with malware from doing anything to your computer.
For cleaning the registry/deleting temp files/editing startup items I like CCleaner I recommend taking a system checkpoint before running this in case something important gets disabled.
For a simple program that does many of these scans all at once I like Glary Utilities Simply click a scan button and it will run Antispyware, registry, temporary files and browser history scan. Once the scan is complete click another button and it deletes anything it found.
For getting rid of unwanted programs that dell/hp/sony/etc install by default I like PCDecrapifier It runs tries to figure out what programs you don't need and shows them to you in a list. You can then check the ones you want to uninstall and it'll try to uninstall them for you. I recommend taking a system checkpoint before running this in case something important gets disabled.
More Info: a PC optimization guide from notebookreview.com