Nuno 3.4 which is our very latest Nuno does not install on Windows XP. Microsoft itself stopped supporting Windows XP in April 2014 (link to Microsoft website).
The latest version of Nuno that can run on Windows XP is version 184.108.40.206. Download it from here.
We'd urge anyone still using it to consider something more modern Windows 10 is much faster and more user friendly.