Condition Zero is built on the half-life engine, but because I'm not an owner of it myself, I wouldn't know whether it's possible to run NS properly with CZ instead of a retail-purchased version of half-life, counter-strike, or day of defeat.

If the beta quick start instructions here can't be applied to CZ, I'm afraid you'll have to pay the 10-15$ for Half-life. Hopefully another CZ owner can come give a more certain yes/no answer about whether you can run NS right now.

[edit]Just realized you double-posted this thread to here as well. Please try to avoid this in the future; such clutter often forces us to lock, delete, or merge duplicate threads.