Are you licensed to do that? It is most likely why you have a problem with it. Think about it. If a single install CD can be copied to a network drive, then, gosh, you could easily run the install onto as many computers as you like. It is true you CAN do that if the computers have a CD drive - but it is not legal to do so.
That being said..., just out of purient interest you understand, how exactly are you trying to copy it? Again stating that your EULA may not allow you to do what you are trying to do.
Although frankly, I don't think you can, but hey I could be wrong. The problem (invalid MS-OS function) could be the fact that the executable archive for WINWORD.EXE is spanned across multiple archives. This is why install programs create temp files. They collect large files from spanned archive and recombine them. So I am interested to know precisely how you are trying to copy.
Gerry