During the initial phase of installation, before the progress bar moves at all, while the steam client states: "creating local game cache files" and "preparing TW: Rome II files for install I get a message box that reads: "Steam - Error An error occurred while installing (unknown error)".
The same "unknown error" occurs whether I attempt to download from Steam or from the disks (following the instructions posted on Steam and here).
I have been all over the forums for an answer and cannot find one. I'd post my dxdiag.txt but I get an IOerror uploading whether I'm using IE or Chrome. I can email it if necessary.
This experience has been very frustrating - please discontinue TW's association with Steam.