This is the No-CD patch or 'crack' for Half Life 2. When applied, there is no need to insert your CD when attempting to play the software. Simply * Install the game - Full Installation.
* Extract the hijack_hl2.exe, hijack_hl2.dll & hl2.exe to the directory which holds steam.exe
* To play the game, execute hijack_hl2.exe instead of steam.exe
and play away - follow the instructions in the file if in doubt!