Call was rejected by callee Word automation

Hi, I have an error that is causing me some grief to say the least;

I have created a vb.Net application that is used to automate printing of certificates.

The application utilises a Microsoft Word 2000 template with bookmarks and simply retrieves the necessary information from a database and populates the bookmarks.

The program works fine for printing on an A4 size piece of paper. When I choose to print on a custom size piece of paper (AS specified in the word template) I get the following message;

Call was rejected by callee

The ONLY difference between the print methods is the size of paper as the Print method simply opens the appropriate template and then populates the bookmarks. A4 prints fine but A5 (Custom size) falls over with the above error.....

I am not using Norton antivirus with the computer as mentioned in other posts....

Help appreciated
Regards
MarkusJ