I've been quite happy with Avast Free, although it has been mentioned to cause issues - which I either haven't noticed, or have been able to take care of.
About the issues: I've witnessed Avast can quarantine a perfectly legit program as a virus. False positive, that is. Simply revert them from the Quarantine and put them on Avast's bypass list. You may also put them on the ignore list before you run into a program that won't start due to a missing .exe file. Just take care you are bypassing only fully trustworthy programs like AH!
Another thing is to make a custom install of it. The most important features are in the left hand pane and you can even safely uncheck the email scanner if you use webmail instead of an email program. The long list in the middle can be unchecked, as you may notice most of them aren't exactly virus killers. That would leave it about as lightweight as MSE but IMO a little more powerful.
