Sorry to come back on this.
I have replaced the HtmlBox with a panel on which I dynamically create LinkLabel objects. The LinkLabel objects are created from a list received from a database of file names. I have also implemented the FileDownloadGateway control as per the wiki and other posts on this forum.
When I click on the LinkLabel for the file, VS2008 breaks at a point in Resources.Gizmox.WebGUI.Forms.Form.js.wgx, where the code says "{window.open(v2,v189,v647)", with an "Access is denied" error. This appears to be a VS 2008 error message rather than ASP or VWG. The options are then to Break, Continue or Ignore. Nothing happens if I click Break or Continue. If I select Ignore, then after a few further seconds, another error appears saying "htmlfile: Invalid argument". If I select Ignore again, the file opens in a new tab.Can anybody help, please?