Hi Runar,
Word has a 'master document' feature, but it seems especially prone to corrupting documents.
A reliable alternative is to use INCLUDETEXT fields to link your main document and its various parts. You can create such a field in
the main document via Insert|Field. Word's Help file provides more details. Once you've got the documents linked, if you're editing
in a linked area of the main document, pressing Ctrl-Shift-F7 will propagate the changes to the source document - if you don't do
this the changes will be lost in both copies. On the other hand, you can edit the source document in the normal manner and, provided
you've got the 'update links' button checked under Tools|Options, the changes will be reflected automatically in the main document
next time you open it.
Cheers