RegDeploy: applets aren't run if old plugin usage is set in 7u4 b13 ( Win + IE browser )

Short desc, see comments for details:

  After switching to old plugin applet desn't work in IE for 7u4 b13
  No such issue for 7u4 b12.

  Note:
     - if FX 2.1 b14 is also installed after 7u4:
           old plugin works Ok first time,
           then after switching new plugin ON/OFF works only new plugin.
     - if FX 2.1 latest scrum build ( b80, Feb 25 ) is installed after 7u4:
           old plugin doesn't work in the same manner as in case of no FX installed