Initial thoughts
That, in a nutshell, is Microsoft Security Essentials. We've had it running on our system for a touch under 24 hours - clearly not long enough to pass judgement - but we're already impressed for a number of reasons.
The software is notably light, it's incredibly easy to install, and it performs admirably. Once up and running, there's little to distract the user - we've seen no pop ups thus far and the software just runs without interfering.
Having made the switch from AVG Free, we've noticed that our test system is more responsive and able to boot into the Windows 7 Release Candidate distinctly quicker. Although far from a definitive test, a look at Microsoft Security Essentials' memory footprint highlights its lightweight nature. Immediately after install, it occupied in the region of 7MB of memory on our test system, and that figure dropped to under 5MB after the first reboot.
It's worth noting, also, that the software acts as a superset of Windows Defender. That pre-installed anti-spyware solution is simply disabled, allowing Security Essentials to take over responsibility.
Only time will reveal how Microsoft Security Essentials will fare against the ever-evolving threat of malicious software, but even at this beta stage it's a promising-looking solution, and it's free, too.
Users in the US, Brazil or Israel, can download the beta at Microsoft.com/Security_Essentials. For everyone else, the beta is also being distributed through Microsoft Connect.