MSE has a few holes in its security as microsoft have to allow the full functionality of Windows specification even though these can allow certain security flaws.
I would recommend "Zonealarm Free Plus Antivirus", keeps the firewall and antivirus in one operation and is a bit tighter than MSE.
I also advise running Malwarebytes Free once in a while to pick up the more obscure things that happen. Malwarebytes is better than most programs but the free version doesn't run in the background (and I wouldn't want it running all the time anyway).
I also run CCleaner before I switch off to clear up all the temporary files which can often contain threats, this avoids the antivirus picking them up on its next scan and slowing things down by doing a full scan. I switch the background checks of CCleaner off, mainly because I don't really know what they do and have no need for it.
All these programs try to trick you into downloading toolbars, browsers or trial versions of these programs while installing or updating them - just say no.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates