All credit goes to Patriots190, he posted it in the rs2.6 issues thread but I still see poeple having problems so I thought I'd make a thread here for everyone to see. Here's what he did, and it worked for me as well.
I found that copying the Steam.dll file from the steam folder to the Rome Total War Gold folder will work, for those who have Steam but are getting this error. One of the solutions from this thread (about vanilla RTW, not RS):http://forums.steampowered.com/forum....php?t=2516059 Apparently it can be an issue with RTW on Steam without mods, not sure why it would suddenly start though.
After copying the dll I was able to load the game and start a Roman campaign without issues.
The modders may be working on a different fix but for now this should get you in the game. again BIG thanks to Patriots190 forum member!!!!