Does HL1 work in Steam? If HL1 does not work in Steam, then you will not be able to play. You have to start a game of HL1 (you can exit immediately after you make the game) before installing NS.
Make sure you installed it to Steam\SteamApps\email\half-life\. If you installed it correctly, there should be a folder called ns inside of half-life, with lots of folders and files inside of that folder. If there are two ns folders with one nested inside the other, then just delete the ns folders and install again.
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Make sure you installed it to Steam\SteamApps\email\half-life\. If you installed it correctly, there should be a folder called ns inside of half-life, with lots of folders and files inside of that folder. If there are two ns folders with one nested inside the other, then just delete the ns folders and install again.