Help please: Master page of web application in _layout folder in MOSS

[bin_bin1]

Hello, there, I have a question about Master page of web application in SharePoint _layout folder. I can create a web application in the layout folder and have access to SharePoint Object Model. However I cannot use Master page for my web application. I tried to copy SharePoint default.master to my working folder and put the content of my page into content holder. When I load the page, I got a SharePoint error. Also I tried to use a most simple master page and content page,I still got the same error. My question is how I can use Master page for the web application in the layout folder? Anything that I need to modify? Also how can I use the most recent default.Master in my web application to keep my web application always has the same style as other pages in the SharePoint. Any suggestion will be greatly appreciated. Best regards, Bin