Where is the game files

[ONI NavSpecWar]

I'm completely lost on this but how do I find the game files to add mods into the game

I'm Using XP and installed through disk if you need to know

Yes I realize I worded the title wrong I was rushing

[AlfonsoVIII]

Most common place (depends on where you installed it to):

C:\Program Files\Steam\steamapps\common\empire total war\data