I have low system memory and work in a pretty safe environment so don't feel it wise to have a big antivirus running in background all the time. I also have multiple systems so a portable AV that I can place in all of them through a USB stick would be the nicest option for me.
I have tried ClamWin Portable, but it is far too slow to scan and its virus detecting skills is on the lower side.
Other antivirus solutions mentioned here and there in discussion forums are less known and I am not sure whether to try them or not.