I do not know about Half Life 2, but Oblivion has a problem exactly like the one you are describing after being patched up to version 1.2. If you are using that, the problem with Oblivion is most likely linked to it. You should also try disabling any mods you might be using, as bad, or incorrectly installed, modification could also cause the black-screen. Videocard drivers are another thing you should check.