MS Security Essentials should be used as the only realtime AV scanner on your PC (and to clarify SD's statement above, it's this which is crucial you don't have more than 1 of, as they fight each other). It's highly advised to install the free version of Malwarebytes as that doesn't have a realtime component and hence doesn't interact badly with any other AV program, and it's extremely good at ridding systems after an attack. You should also use the Windows firewall as this completes the set of protection. You shouldn't however need Windows Defender, but as explained above it doesn't have any realtime scanning portion and hence can coexist with other things if you so desire.
You won't be 100% protected, but then nobody is 100% protected, but you'll give yourself the best chance of finding a virus or malware, the best chance of eradicating one if it gets onto your PC, and most noticeably of all the best experience of your PC whilst it's operating properly
(and apologies to everyone for the dictatorial nature of the above, but it's the quickest way of getting the message across!)