Well, since your system has lower memory, I would advice you not to opt for the high end anti virus solutions. They are heavy on your resources and having them with real time protection will considerably slow down the performance of your device.
A few of the best options for you would be -
It Is one of the quality anti virus solution available. It comes with an exciting set of features. It also provides you application analysis and such advanced features.
It has been voted as one of the best by users. It does not come with many settings. Quite minimalistic in design. Perhaps you will not like it if you want to fine tune the security settings. It has no scans, no settings or no scheduling. Once you install it, it just takes over and handles the real time protection.
3. ESET NOD32
It is one of the effective anti virus program with some good extra features. The software does not interfere with your day to day functionality of your machine. It does not have anti spam filter though.
Live....and Let Live!