This issue has been occurred after I had updated tools (26.6.2)+ migrating to win7 from xp.
The possible reason perhaps in proguard place because some folders can have a space in their path folders' names(like "Program Files",e.g.).
In my case i did what has been pointed above but copied the entire proguard folder to new place (mine new became "c:\proguard") and put its path to PROGUARD_HOME
SET PROGUARD_HOME="c:\proguard"
in $ANDROID_SDK\tools\proguard\bin\proguard.bat file.
Another possible way:
After tools upgrading i found very simple solution:
find environment variable
PROGARD_HOME on windows
and edit its path to
(in mine case ,currently) :
not C:\Program Files\Android\android-sdk\tools\proguard
but "C:\Program Files\Android\android-sdk\tools\proguard"