This is from http://www.readyroom.org/ it seems there is a prob with retail users..
Dunno what it is, cos Im a mod player, BUT. This is a fix.
News and Fixes and NS, Oh My!
Man oh man, NS has really hit the big time. Even with a tiresome CPU utilization issue, it's the #4 HL mod right now, catching up with TFC and DOD very quickly. Once Flayra gets the server-side patch ready (hopefully before the end of the weekend) look for even more servers to start up.
Fix for Problems Installing with CS Retail
If you are running a copy of CounterStrike Retail (usually a non-English version), and are having problems installing NS, we have a workaround for you. This seems to happen to a handful of users, but the fix below will get NS onto your system:
1. Download the dummy half-life executable file from here
http://www.natural-selection.org/downloads/hl.exe
2. Copy file into your counterstrike retail directory (in the same spot as your cs.exe or cstrike.exe)
3. Run the Natural-Selection Installation
4. The NS installation will think the dummy HL.exe is patched correctly, and complete your installation.
Now, there is a caveat to this - the installer will create a shortcut that points to the dummy HL.exe. You will need to edit your 'Lauch Natural Selection' shortcut to point to your counterstrike exe. That is the only downside. Once you install, you can actually throw the dummy HL.exe away.