Well, informing us that your running Vista makes giving an answer much easier (not kidding/no sarcasm).
For Vista (&Windows 7), it's here- C:\Users\[username]\AppData\Local\Ironclad Games\Sins of a Solar Empire\Mods-v1.17 (or Mods-Entrenchment v1.03).
AppData is a hidden folder. Installation may vary; for some, you may need to get 7Zip or WinRAR, but for most just Windows is good (native un-ZIPing is so good). So you'll need to extract the mod files; CHECK THE FOLDER HIERARCHY!!! This is actually a very common thing to do; if the folder hierarchy is 'bad' then the mod doesn't work. You'll want the mod to have only one main folder, [modname], followed by a 'GameInfo', 'Mesh', 'Textures', 'String', 'Particle', 'PipelineEffect', or similar (not all are necessary to be present).
Installation is simple; just drop them in the mod folders. noted above (Mods-v1.17&Mods Entrenchment-v1.03).