I have been using Microsoft Security Essentials (MSE) for quite a while now and I am very happy with it. MSE is free to download by the link Joe M provided. MSE has not interfered with any of the programs I run.
The settings in MSE allow you to disable real time scanning for threats if you need to for working offline when you need additional resources. However I never do that even when running AutoCAD and Land Desktop software. Any good anti-virus software should allow you to control how and for what it is being used.
I am not sure if MSE will monitor software that you have installed on your computer to see that it is a valid and licensed software. I have read that it does monitor for illegal software, however, I only install software that is personally licensed to me anyway.
Best of luck with your decision.