At this point you'll have to download the now free beta Bootcamp from Apple. Google "Bootcamp Apple" to find the link. It's pretty straightforward, but you'll have to have a copy of Windows XP with **Service Pack 2**. Bootcamp will walk you through installing Windows.
When you're finished, you'll have the choice to boot into Windows or boot into OS X. When you boot to Windows, your Mac becomes a Windows machine pretty much like any other Windows machine. And it should play Half-Life 2, etc. very well.
Let us know if you run into snags.
Doug
P.S. There is experimental 3d graphics support in the (free for now) VMWare beta. I don't know if HL will run.