Avast offers both a free and paid version of their software. While the free version is very functional, consider purchasing the commercial version as the additional functionality offers considerable protection.
The free version of this software does not offer real-time protection, but scanner is a great tool for after-the-fact infection cleanup.
Super Anti-Spyware offers both a free and commercial version. The free version does not offer real-time protection but is a good after-infection scanner. We often use malwarebytes in conjunction with this product as our first tier cleanup effort.
This utility is not for the neophyte. It allows the user to disable execution of viruses and malware, but does not remove it. A dirty little secret of computers is that a virus can reside on the machine but can only do harm if executed, so disabling execution is often the first step to removal. WARNING: You can cause serious problems by improperly using this product.
This program offers real-time protection against registry changes as well as the ability to scan for malware. Intended for the more technically inclined, we usually recommend it for post-infection cleanup unless you rarely install new applications, as SpyBot is very aggressive about prompting when changes are seen.
Clamwin offers both a free and commercial version. The free version does not offer real-time virus protection of the computer, but does offer real-time scanning of email if you use the Microsoft Outlook email program. For those that don't like real-time protection, this is a good alternative. Keep in mind, no real-time protection means many viruses and malware will be caught after the fact.
Panda has a quality product (both free and commercial), although not as popular as many others. It offers both virus scanning and real-time protection.
This is a great anti-malware tookit, but has a very rudimentary interface, essentially a dos-window. When all else fails, this is worth a try.