Stop Microsoft Word re-using WinWord.exe process created via COM Interop

Open Component Services (I'm not sure where this is for Server 2003, but probably under administrative tools).

Expand to Component services->My Computer->DCOM Config.

Find the application you're wanting to connect to, right click and go to properties. Under security, you should be able to set launch, activate and access permissions.

You'll need to click the Customize radio button before you can edit the permissions. You'll have to add any users you want to both Launch and Activate, as well as Access.

(All of my advice is adapted from this answer about Excel interop.)

There are a few important things here:

1) Never use 2 dots on the same line. Also consider an indexer as a dot

Good

Word.Documents d = wordApp.Documents;
Word.Document aDoc = d.Open(/*...*/);

BAD

Word.Document aDoc = wordApp.Documents.Open(/*...*/);

2) Release all of your pointers.

3) No really, go back and release all of your pointers, you missed one somewhere (or at least I always do).

Here's a full example of what FINALLY worked for me on one project after much wailing and gnashing of teeth:

object m = Missing.Value;
// this must be an object, not a string. if you forget though,
// intellisense will remind you
object oFilename = @"C:\my sheet.doc";

object readOnly = false;
object isVisible = false;

Word.Application wordApp = new Word.ApplicationClass();
wordApp.Visible = false;
// remember: don't use 2 dots on 1 line
Word.Documents d = wordApp.Documents;
Word.Document aDoc = d.Open(ref oFilename, ref m, ref readOnly, ref m,
    ref m, ref m, ref m, ref m, ref m, ref m, ref m, ref isVisible,
    ref m, ref m, ref m, ref m);
aDoc.Activate();

object findText = "my old value";
object replaceText = "new and improved value";

object oTrue = true;
object oFalse = false;
object replace = 2;
object wrap = 1;

Word.Selection s = wordApp.Selection;
Word.Find f = s.Find;
f.Execute(ref findText, ref oTrue,
    ref oTrue, ref oFalse, ref oFalse,
    ref oFalse, ref oTrue, ref wrap, ref oFalse,
    ref replaceText, ref replace, ref oFalse, ref oFalse,
    ref oFalse, ref oFalse);

aDoc.SaveAs(ref oFilename, ref m, ref m, ref m, ref m, ref m, ref m,
    ref m, ref m, ref m, ref m, ref m, ref m, ref m, ref m, ref m);

object doNotSaveChanges = Word.WdSaveOptions.wdDoNotSaveChanges;
// casting here because intellisense complained of ambiguity
(aDoc as Word._Document).Close(ref doNotSaveChanges, ref m, ref m);

// release each in the reverse of the order in which it was first used
// ReleaseComObject might also work as well. I haven't tested yet
Marshal.FinalReleaseComObject(f);
Marshal.FinalReleaseComObject(s);
Marshal.FinalReleaseComObject(aDoc);
Marshal.FinalReleaseComObject(d);

// must quit app before releasing
// again: casting because intellisense complained of ambiguity
(wordApp as Word._Application).Quit(ref m, ref m, ref m);
Marshal.FinalReleaseComObject(wordApp);