Half-Life

The most important thing the clients need to play Half-Life and it's mod's is the Half-Life CD, and the CD-KEY which is provided on the case of the CD. After installing the plain Half-Life versions, you first need to update it to the latest 1.1.1.0 version which can be downloaded here.
 

Counter-Strike

Download the latest Counter-Strike 1.5 release right here.

Install in your Half-Life folder, follow the instructions and play the game.
 

Day of Defeat

Download the latest Beta Day of Defeat release right here.

Install in your Half-Life directory, follow the instructions and play the game.
 

Frontline Force

Download the latest Frontline Force release right here.

Install in your Half-Life directory, follow the instructions and play the game.
 

Beta Counter-Strike

Installing a Counter-Strike Beta release is very simple. Instructions on how to do this can be read on http://www.langames.net

Or you can go for a manual install. In that case it is necessary to download the original files.

The Half-Life 1.0.1.6 update can be found here.
The small Half-Life 1.0.1.6 fix can be found here.

You can download Counter-Strike Beta 5.2 here.
And download Counter-Strike Beta 6.1 here.

When you play Counter-Strike Beta 5.2 or Counter-Strike Beta 6.1 don't forget to do the following commands in your console:

fps_modem 72 ( sets FPS to max, which is 72 with this release )

rate 6000/1000 for ISDN/cable or xDSL connections
rate 12000 for T1 and greater connections

r_shadows 1 gives the models real-time shadows when they walk or move.
lambert -1.1 gives all models some kind of glow, which makes enemies easier to detect in dark levels.

* Don't forget that if you want to play Beta 5.2 or Beta 6.1 you have to install the 1.0.1.6 Half-Life release, and NOT the latest 1.1.1.0 Half-Life release.
 

* Note : all client downloads mentioned above, are all FULL versions, and are all for Windows clients.