All the files for modding SoaSE require 2 programs.
And image editor that can handle the alfa channel and DDS file types. Gimp is free open source and it work s quite well.
Gimp: with the DDS plug-in. Use any image editor that can edit the .dds format. This will require to download a DDS plug-in for most image editors.
A text editor like notepad. I recommend Notepad++. Most text files will end with .txt which will auto open notepad but other file types like .entity or .brushes or .window or .str will need to be told what program to use because windows does not recognize those file types.
Right click the file you want to open. Choose "Open". A pop up box will appear and select the second option.
"Select a program from a list of installed programs"
Choose browse and point to the Notepad.exe or Notepad++.exe.
That file type will now have different icons and always open with the program you picked. You can change the program later by right clicking the file and selecting "Open With" and choosing "Default Program" then select the program you want to use when double clicking that file type. This works for all file types like music and images as well.