The problem can be almost anything *if* the pc's are not mirror images of one another. I use automation to read/write to Excel and one problem I encountered was that the test machines, the user machine and my machine all had different SP levels of Excel. Oddly enough, the test machines version fell between mine and the user and they were the only ones experiencing the problem.

Step 1 ensure that all machines are running the exact same version of Word.
Step 2 examine the .dll's used by the different machines (feel free to curse Microsoft while doing so)