Let's say that the most recent patch has broken your game. Or perhaps you've made changes to a vanilla CA file only to discover that you're not the man you wished you were: all your hard work leads to CTD after CTD. In any case, you'd like a clean slate. The process is daunting, however, as reinstallation takes hours, which is all the worse if you live somewhere with metered or capped downloading.
I'm pleased to say that you needn't fear. There's a simple solution. Steam can return your game to pristine state without too much trouble. Here's the procedure:1) Open Steam.
2) Right-click on NTW and select Properties > Local Files > Verify integrity of game cache.
3) Steam will search your files and replace only missing or corrupted files. Viola! That's it. No need to reinstall. [CAVEAT: That also means that any desired changes to a vanilla CA .pack will be erased. If you have a modded CA file that you want to keep, back it up or rename it and then replace it when Steam is finished.]
Wait, you say, things are royally messed up and even this doesn't fix my problem. What now? If that's the case, do this after you do the above:4) Open your NTW data folder (you can find this quickly by typing "data" in the Start button search menu).
5) Remove every modded file that you see. If you're not sure, delete it anyway. Remember that you can delete a vanilla file and replace it by repeating steps 1 - 3.
6) Click on the Start menu and search for "user.script". Delete it.
At this point, your game should be 100% vanilla. If not, then you may need to reinstall. However, always try the steps listed above first as they will solve your problems 99% of the time while saving you hours of frustration and GBs of bandwidth.