My top list of reputable Anti-Malware/Virus suites for Windows that have free editions or fully functional trials.
They're in no particular order of effectiveness at the time of this writing . . . these are all genuine and are usually listed within the top 10 AV products as tested by VB100. I am posting this as a reference because there are way too many pop-up ads for so called free scanners that are actually Trojans in and of themselves.
Remember that you should only run ONE real-time protection product at a time on your system. Don't install two or more and expect your computer to be stable.
Links provided in clear text so you can examine them for funny business.
SunBelt Software: Vipre - 15 day free trial. (Fully functional, Virus, Rootkit, Malware/Spyware protection and cleanup. Very useful for emergency cleanups.)
ESET NOD32 AV - 30 day free trial. (Mostly fully functional, Virus, Malware/Spyware protection and cleanup.)
Kaspersky Anti-Virus 2009 - 30 day free trial. (Mostly fully functional, Virus, Malware/Spyware protection and cleanup.)
Sophos AntiVirus - 30 day free trial. (Fully functional, Virus, Malware/Spyware protection and cleanup. Free Rootkit analyzer also available, see below.)
Sophos Anti-Rootkit - Free version. (Fully functional within the scope of the intended use, that is to find and delete rootkits - but it's not going to go after other malware or viruses on your system.)
Avira: AntiVir - Free version. (Good protection and system scans, but pops up nag screens from time to time asking you to upgrade to the pro version.)
Avast!: Home Antivirus - Free version. (Good protection etc, free virus definitions seem to be about 4 days behind -- but I cannot prove that.)
Grisoft: AVG - Free version. (Good protection etc, as with Avast the free virus definitions seem to be about a week behind -- but I cannot prove that.)
Since someone may ask -- I personally use the first on the list. It provides excellent scan and cleanup features including a special safe mode scanner and a boot-time rootkit scanner. Its real-time monitor has very low impact on system performance and the program has a very clean -- even simplistic -- UI.