I'm having the same problem and it only just showed up about a month ago.
Oddly I'm having the problem with Vista and Windows 10 and not XP. In fact, xp plays nice with SL viewer: still.
I will grant that xp uses an older version of SL because they don't support it any more.
But Win 10 was more explicit about what had caused the DEP to activate and it appears that it is the plugins that are causing this; incidentally I see the plugins often trying to spawn endless plugs and so when I have been diligent about stoping them I have been able to keep logged in and trouble free.
So the questions become more of :why are the pluggins not getting cleaned out properly and how are they producing the buffer overruns that look dangerous to DEP and why is it that when trying to call out the Secondlifeviewer.exe as an exception Windows vehemently denies it that privilege.
It all stacks up to look like it is a big problem and one that the OS is well aware of and wants to protect the users from.
I too am surprised that this is not a problem for a multitude of users.
It would possibly help more if SL developers were more forethcoming about what exactly goes on with SLplugin.exe llceflib_host.exe and win_crash_logger.exe since they are the ones causing these overruns and they have a number of dependencies that might well be a part of the problem or a key to understanding the problem.
It's even more remarkable that there hasn't been an extensive conversation about this with a call for more transparency as regards the benifit and necessity of these pluggins and how that outweighs the trouble that they are causing or for that matter - why it is that each developer has their own set of these that require anyone running more than one viewer to endure the effect that several divergent instances cause.