Shoutbox

Or, if you don't want to install Steam, use 7-Zip to extract the "portal prelude" folder from the installer.

Rename it so there's no space to something like "portalprelude" (otherwise you should add quotes around the path in the next step).

Then create a new shortcut to hl2.exe like SonicSam said and add -steam -game <path_to_prelude_folder> to the end.