Sounds like a stupid question, but anyway; which one is newer ? 1.0.2140 or 1.0.1501 ? 1501 just leaked. 2140 leaked a couple of months ago.
When MSE was released to public, the version number was "1.0.1407.00". That has been superceded by the latest publicly available version "1.0.1501". Any other version should not be used and is either an internal test version or a pre-beta. It is always at your own discretion whether to use leaked versions of anything but considering what you use MSE for, I would suggest sticking to what MS provides as "publicly available" builds os that you don't leave yourself open to security risks. As with W7, there are likely different "build branches" so a higher number in an un-released branch may not necessarily be a newer version. If I recall correctly, 1407 was actually a newer (more recent) build than 2140....
The Time Stamp of v1.0.2140 full installation package is 2009-06-14 The Time Stamp of v1.0.1500 update package is 2009-07-08 Although v1.0.1500 is released later than v1.0.2140 but v1.0.1500 update package does not upgrade v1.0.2140 and it says: "A newer version of Microsoft Security Essentials is already installed on your computer." I think v1.0.1500 is the upgrade for v1.0.1407 It's some what strange to me.
Nope. Build number is from some unused branch or something. Latest build, no longer in beta and officially released today, is 1.0.1611.0.