You either need an archive (.zip) version of the patch, or you can do it the hard way and uninstall RTW, then go through your registry and delete all activision and rome total war references. I don't recommend that if you are not an experienced user, you can do damage to your installation.

Perhaps some kind soul will post the .zip version of 1.2 for all to download.

By the way, I am moving this to our technical support area. I hope to keep the forum relatively organized.