In Windows using C#, how can I get the installation path of a software (for example consider NUnit or any other software like MS word, etc.) from my project
Use the system and application classes. This will give you all sorts of information.
EG: Application.ExecutablePath
It also provides methods to do what you want to.
Edit: Also see registry read/write instructions here:
http://www.c-sharpcorner.com/UploadFile/sushmita_kumari/RegistryKeys102082006061720AM/RegistryKeys1.aspx?ArticleID=0ce07333-c9ab-4a6a-bc5d-44ea2523e232