Can you try to do the following?
1) Update your GPU drivers.
2) Remove the game manually from your PC by going to the folder where Steam downloaded 'Holdfast NaW' and press delete.
3) Log out from Steam.
4) Log back in on Steam.
5) Download 'Holdfast: Nations At War'.
6) Verify the integrity of your game files through Steam.
7) Launch the game again, switch the graphics to a 'Very Low' Preset' and try joining a server.
PS: If you have an anti-virus software, I would also suggest that you temporarily disable it as it may be conflicting with the game.
Let us know how it goes.