Your computer weighs 50lbs? That's crazy. There are cracks for steam, but again they require steam to be connected to the internet atleast once. You'll need to download the nosteam version of Portal instead.
In any case, does the download and go offline thing really work for any game? ; I downloaded a 'crack' for steam before but it just sucks, a lot. Doesn't let me download every game, and has problems with some, -tries the offline mode-
Get an emulator called UCL (Universal Content Launcher). Google it, you'll find it. Don't use SteamEmu, its purpose is to make non-steam standalone games, what you want is an offline Steam replacement emulator/GCF launcher. You can also try Steam-Buster Revolution Orangebox, although it is no longer updated, UCL has more features, etc. All of these are virus-free, I have been using many of these for years.