When building an ASPapplication that generates Word documents,  it was working fine on the local machine. A windows XP installation with Office 2007. When we deployed it to windows 2008 (64 bit) and Word 2007 we saw that Documents.Open returned null. Other than this, there are no exceptions.

The code that returned the error (simplified):

Application app = new Application();
Document doc = app.Documents.Open(ref name);

As it turns out, a 64 bit system stores temporary files in “C:WindowsSysWOW64configsystemprofileDesktop” folder.

The solution: create a new directory called “Desktop” inside “C:WindowsSysWOW64configsystemprofile”

Hope this helps.