If the downloading has started, then as far as I can remember, you can't stop the download and continue gaming. You must finish the downloading first. Or, you can uninstall the game and re-install it and skip the Hi-res Texture Pack to get to the game.
"Take your best shot, Flatlander woman". JC Denton.
try going into offline mode and deleting whats downloaded of the dlc, to see if it lets you start the game then, or if youve got a freind locally with the game and dlc installed they could copy it to a flash drive, so you can add that after closing Steam, restart Steam and see if it accepts it