You can try downloading the non-Steam version from TWs website (see here:
https://www.taleworlds.com/en/Games/Warband, first entry in downloads section). You will need to activate it using a serial key - you should be able to use your steam one (Library - Warband - Right Click - Manage - CD Keys).
If that key doesn't work you may need to convert it to a different format - I believe theres a tool for that on one of TWs sites somewhere but I'd have to search for it, so please let me know if you do need it.
