Everyone who's experiencing issues with Counter Strike Condition Zero should reinstall the game or just following the instructions below:

First of all, please download this two files below:
(Schlachthaus isn't responsible for the content provided in these links)
counter-strike.zip
gcfscape182.zip

After that, please extract them to your desktop, install gcfscape as it is and start the program by navigating to:
Start -> Programms -> Nem's Tools -> GCFScape

Once, you're inside the program, please navigate to
File -> Open

and open the extracted file: counter-strike.gcf

Then highlight the folder cstrike, right click and...
extract

choose your desktop as target and hit...
OK

After you extracted the files, you should be able to see a cstrike folder on your desktop!

access this folder, highlight all files -> right click -> copy

and navigate to your steam folder:
Steam -> SteamApps -> common -> Half-Life -> cstrike

now right click -> paste

Not all existing files have to be overwritten... if you do so... you don't do anything wrong!
Now test, if it works for you! Start the Game and test it...

Just in case there's an error which tells you that the "player.mdl" is corrupt, then navigate to your desktop again to:
czero->models

find and copy the file player.mdl and overwrite it in:
Steam -> SteamApps -> common -> Half-Life -> cstrike -> models

NOW you should be able to start the game again without any error messages. Please, also double check every change in the following folders:

Steam -> SteamApps -> common -> Half-Life -> czero
Or
Steam -> SteamApps -> common -> Half-Life -> czero_german

Now, copy your config file from your old czero/czero_german directory and paste it in again.

Hopefully, we could resolve your issues, and keep your old data without reinstalling Condition Zero. Valve is going to fix this problem (hopefully). Just in case there are some issues again, we will take care and find a solution again.

Source: CS Attack Roland (retranslated into a better English).