What he said^^^. A lot of cracks also come with a .txt or .nfo file with brief instuctions on how to install it. Some require you to run a "crack.exe" (or something similar) from the same directory as the game files, others are just simple file replacement. I would install off of the original store-bought copy of the disk and then crack. It's not a bad idea to back up the original .exe file before you replace it, too.
For Blizzard games be aware that two of the same product key cannot be on Battle.net at the same time. LAN multiplay should work though.
<font color=blue>Insanity is doing the same thing over and over again and expecting a different result.