Downgrading to an earlier version isn't good if one of the reasons for the upgrade was a security update.
The new version doesn't work, so the old one is better.
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Version 4.8.1351
August 17, 2009
improvements in some unpackers (WinExec, Installers, Droppers)
various fixes and improvements in the scanning engine
Standard Shield: stability improvements in the kernel-mode drivers
preparations for smoother migration to v5 (when it's released)
Version 4.8.1335
February 5, 2009
....
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Where is "Security update"? It's about stability improvements. I only see "fixes" in the scanning engine (not so clear what it means, but I don't think security updates).