So first install the Wine PPA (Does not matter if you already have PlayOnLinux installed). To play both, HL and HL2 (Not through steam) you first need the Wine PPA (It is much better than the actual version that comes with Ubuntu). Also know that Half-Life is natively supported in Ubuntu. This one only apply to games outside the Steam environment or not yet on steam.
If you came here to play HL, HL2 or any other Steam for Linux released game see How can I install Steam? for the answer which is a lot simpler than this.
