mIRC Help file outdated for mIRC 7.1
Error:
$os
Returns the version number of the operating system. The reply can be 95, 98, NT, ME, 2K, XP, 2003, or Vista.
Correction:
$os
Returns the version number of the operating system.The reply can be 2K, XP, 2003, 2008, 2008 R2, Vista or 7.
I believe it should be like that due to the dropped support of 95, 98, ME, and NT.