Save game at the end of the infamous cave sequence at the start. Drop the 2 files into _appdata_\savedgames or wherever your save games are. Load the save and talk to the NPC closest to you, then drink what he gives you. You'll be dropped back in Cordon where you can now talk to Sid as usual. Be sure to also inspect the new corpse by the truck near the tunnel with the broken jeep in it, just before where you first meet Petruha - this will start the main new quest chain.