I use free Avira as anti virus,SAS and MBAM as anti malware as malware scanners and Spywareblaster to try to block malware and it also helps to block adverts too.I use the free versions. If in doubt uninstall the SAS and MBAM then re install.The free versions of the latter do not work in real time. Avira has signalled that a virus attempt was blocked and got a good review in recent surveys. Others prefer MS Security Essentials, but there have been remarks recently about the difficulty of uninstalling it.
Best Android apps 2015/2016: Best apps for your new phone or tablet - what apps should I download?