Here's a comparison of the top 6 anti-virus'. It appears Bit Defender is free for business, commercial use and it seems to be top rated. I've been using for about 6 months and it's been good to me. It says you can't run an on demand scan, but you can by right clicking the icon. Very unobtrusive and light weight as well.
http://dottech.org/14151/windows-bes...-avast-vs-avg/