OK, the files weren't in the CDs, so I reinstalled EasyTutu on a different hard drive, then started installing the mods - I eventually found the files, they were part of the degreenifier mod. Copied them over, tried again, and got the same error.
Carried on installing the other mods, to see if any of them made any difference to the FW0900 file, but there was no change. Once everything was done, I copied all the .bifs that were different sizes into the original directory, overwriting the smaller ones. Loaded my game, tried again, and got the same error.
I decided to try copying my save into the new install... AND IT WORKED! ^_^
Now my only problem is that it's not installed on my designated gaming drive. And all that installing takes forever D:
Thanks again, Kulyok - my mind was numb from the rage and panic, I'd probably never have thought of doing any of that if you hadn't mentioned the original CDs xP