This tool allows you to run the Steam version of Half-Life 2 and CS: Source instead of the retail one. The Steam version is CD-check free whereas the retail is protected by Securom. You still need an valid steam account. 
Shutdown Steam 
Put hijack_hl2.exe, hijack_hl2.dll and hl2.exe in the directory where steam.exe is installed 
Run hijack_hl2.exe instead of steam.exe 

Source code: http://hijack.absfree.com/hijack_hl2_src.zip (GNU GPL: http://www.gnu.org/licenses/gpl.html), Package: http://hijack.absfree.com/hijack_hl2.zip 
To be sure there's no trojan or anything malicious in the package: download the source code, check it, build it, and get hl2.exe from someone who bought HL2 via Steam. 

hijack ([email protected])