Go to somewhere like Gamecopyworld.com (like TheEnd said) and find the right version that you have. It usually tells what version you have on the disc/package somewhere, or else on the title screen in one of the corners. If not, navigate to the game's directory, and then hover over the exe file, and see if a version number comes up in the hover text. Then just go to gamecopyworld.com and search for "Starcraft" and then look for your game's version of a No-CD/DVD patch =). Done it numerous times with all of my PC games, because I don't like to have to put in a disc all the time xD.
I believe it's LEGAL if you have purchased the pc-game, if not, it's another story.
Yes, its perfectly legal if you own the original game and want to patch it. No worse than installing a game mod to it (because essentially, that's what it is).