Hello 
I am just curious …
What are the difference between avast! Free and Microsoft Security Essentials,
if you compare the use of resources on the computer ?
well i went through and tested a bunch of free ones to find one that had lower resource usage and between those two:
Ram / Page File
Avast 664 / 533
MSE 720 / 623
No AV 648 / 513
running Vista Home Premium(x86)3gigs of ram with an amd turion x2 and those were my results, idk how well they would do for yours.
before anyones asks yes my system shouldnt have any problems running pretty much any av but when you play graphical games or fps you usually go with the lower one and the one you like the best performance wise.
Something way more scientific that the above post: AV comparatives antivirus Performance Tests Dec 2010 (PDF)
I don’t think that you can reasonably compare the to in regard to resources as they are too different in the levels of protection, shields, etc. But, many have found avast to be lighter in regular use than MSE (less of a performance hit, could indirectly be resources).
Whilst the AV Comparatives gives a good overall idea and considers both avast and MSE to be equal, this doesn’t always seem to reflect real life and some of the experiences recounted in these forums.
It’s not easy to compare as MSE has a lot of less features, less configurable, less updates, etc.
MSE has somehow a good detection rate. Which always surprise me coming from MS as they’re major OS manufactures, not antivirus.
Well they do gather a lot of data from their malicious software removal tool.
I just say one thing:
On an old laptop with 256MB RAM, I installed MSE and laptop took years to even open My-Computer, uninstalled that and installed Avira, yet slow (better than MSE anyway). Installed avast 5, things are better and after one reboot even better. I think this will give an idea who is better on resource.