Counter strike Invalid Cd key Error

Often cs players get this error...and this error is common in non steam version only..
But there are always ways to fix the error..Its not that much difficult but it depends upon how do you get this error..
First one being that the game you are using is a standalone installation of counter strike 1.6 only and you have not applied any patch on it but the server HLDS is running a patched version of the game. The best patch that I have come across is the cs16patch_full_V23
The possible ways to fix this error are,

1: Download a fresh copy of counter strike
2: Change your patch or reinstall your current patch..
3: The other factor that could be responsible is that the server needs to ensure that the secure Valve Anti- Cheat checkbox is unchecked.

I hope this will solve your problem..

Regards,
Cs-utilities admin