Windows: Some people have been hesitant to install Microsoft Security Essentials because you can't update the definition pack automatically without Windows Update. If you like manually patching your system, you can easily do so with Security Essentials too.
If by choice or by policy you don't have automatic update enabled, you can still keep your Security Essentialls installation up to date. A quick visit to the Microsoft Malware Protection centre is all it takes to grab an executable to take care of your updating needs.
While you're there you can even take a peek at the update log to the right of the download link and see what trojans, backdoors and exploits are out there making life miserable. Visit the link below to grab the most current update.