turns out the software conflicting with HL2.EXE is STEAM.EXE :phttps://support.steampowered.com/kb_article.php?ref=2275-UFHC-2473
As the error message says by itself there is some kind of a conflict with another software. Check what you have running on a background. Troubleshoot by shutting down all background applications one-by-one until the error is gone.