Which free version of these anti virus programs are the best at detecting and removing viruses on my PC?

I have had good results with AVG.
That being said an occasional scan with an online scanner also helps.
Trend Micro HouseCall - Free Online Virus and Spyware Scan - Trend Micro USA (http://housecall.trendmicro.com/)
and
ewido - anti-spyware and anti-malware solutions (http://www.ewido.net/en/onlinescan/)
are good online scaners

I am also using AVG at my home and it's working perfectly all the time.
If you want you can also try. The avast also i used before it was good and scan the memory properly this the best feature of this anti virus.