I'm jolly pleased with Microsoft's offering. I installed it on the ageing P4 2.8Ghz 1Gb RAM XP SP3 'family' computer previously running the latest from AVG. If anything it has significantly improved boot times as well as picking out and eliminating a Winfixer threat on its first scan, something clearly missed by AVG.
The initial updating process was somewhat lengthy but not noticeably different from clean installs of Norton, Sophos or McAfee, we run alsorts in this house.
The interface is basic and it is lacking some of the bells and whistles of the paid-for suites but I'm confident in its performance to guard the downstairs machine. The question is now do I renew my 360 subscription in January.