In a relatively unadvertised event, Microsoft released version 2.0 of their anti-virus product, Security Essentials. Ars Technica has a write up on the release but the primary change is that the detection engine is now heuristic based in addition to signature based.
In a nutshell, signature based protection can only detect known threats which have a particular "pattern" identified by their signature, whereas heuristic detection allows the system to pick up potential threats based on behavior. Obviously, this provides better protection but at the risk of potentially more false positives.
Overall, however, Microsoft Security Essentials gets very good reviews so if you want to download the newest version you can get it now. It has not hit Windows Update yet but I imagine it will in the near future for users running the previous (non-beta) version.
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