Um, sorry but MSE (Microsoft Security Essentials) has replaced Windows Defender and anyway most good, including free versions of AVs actually turn WD off. It comes with Windows 7 and Vista, but you can install it on XP. MSE is much better than WD anyway but just not good enough itself either, however MSE doesn’t play well together with Avast! or other AVs and some A-M/A-S programs too, although they’ll still run with MSE but overall MSE slows down both XP and W7 horribly anyway, especially even more so noticeably lagged with more on there with it. This is because Microsoft intend it to be a solitary single security standalone, but since it isn’t good enough it has become mutually exclusive with other AVs and security suite software, hence why are we on the Avast! forum?

WD isn’t good enough to rely upon with Avast! which is why the OP is looking for a couple of other, better A-M/A-S programs to complement Avast! Ho.
See history of WD here: http://en.wikipedia.org/wiki/Windows_Defender

The still disappointing and non Avast! friendly MSE actually replaced both WD and the commercial Windows Live One Care, and also turns off WD.
Look all over the net and here: http://en.wikipedia.org/wiki/Microsoft_Security_Essentials