Try running hg -y debuginstall
from the command line. It should show something like that:
Checking encoding (cp1252)...
Checking installed modules (D:\Kazan\mercurial\library.zip\mercurial)...
Checking templates...
Checking commit editor...
Checking username...
No problems detected
If it doesn't, it should give you an indication on what's the problem. If the command is not found, install mercurial from https://www.mercurial-scm.org/wiki/Download or install tortoise-hg from http://tortoisehg.bitbucket.io/