Dotnet interop - Word hangs at Documents.Open, error message "There is insufficient memory. Save the document now."

In our c# application (dotnet framework 1.1), we have 2 customers who have a problem at the following code :
Word.Application app = new Word.Application;
app.Documents.Open(...); // error message : "There is insufficient memory. Save the document now."

This only happens when Outlook is opened.
When we close Outlook, it works fine.
Both pc's use Office 2003. They have more then enough RAM, disk space etc. (all other applications are running perfectly, even Word and Outlook).
On all other pc's using Office 2003, everything works fine (Outlook being opened or not).

What's more, the error just appeared in last week's update of our program.
If we go back to the previous version, everything works fine. But : the code for that part of the application did not change, so we think it has something to do with memory usage.
The application uses around 90 MB of memory (in task manager).

What could be the source of this problem ?
Thanks