Have been using it for a while now. Very light but the malware protection isn't very good anymore.

So now I've installed Avast! Free. Very good malware protection, including basic rootkits. On-demand scan very good as well as the realtime protection (even includes a mail scanner). Also runs very light.

Avira Free is also a possibility. Malware protection a tad better than Avast and also very light. No email scanner though.

If you want MSE, add AVG Linkscanner with it for the webprotection.

Other than that, use Malware Bytes Anti Malware and then the fast scan. Very thorough scan and free. Doesn't provide realtime protection so should be considered as an extra safequard once a week.

Once a month, you should use the very thorough and extensive (free) ESET Online scan.