On a completely updated Windows7 workstation you may have .Net Framework 4.6.
This sometimes doesn’t work with particular (older) version of Mamut Business Software.
In order to fix this, remove all updates for .NET framework from Windows Updates.
Then go to Control Panel, Programs and features, and remove all mentions of .Net Framework 4.6 (including language packs).
Reboot. Now you can install your Mamut software.
(just not tested if it breaks when you update afterwards, but we will see soon I guess)
Hope this helps you,
cheers.