I just downloaded NS. Everything seemed to be downloaded correctly but whenever I try to join a server it doesn't let me and a message appears that says "Client.dll differs from server". I tried redownloading everything but nothing seems to help. Does anyone know what I can do to fix this problem? Thanks.

Here is what you need to do:
1) Uninstall NS completely.
2) Start up steam.
3) Obtain HalfLife.
4) Run through HL's training map.
5) Install IN THIS ORDER!:
NS version 3 Beta 1
NS V. 3 Beta 4
Beta 4a patch.

Enjoy.

In the future please use search - this problem gets reported just about every day.