If you are using Windows XP, you might be affected by several security-related changes that Microsoft has applied to the operating system.
The first is a change to Windows Internet Explorer. When you successfully launch the attachment, the browser might present the following error: "To help protect your security, Internet Explorer has restricted the file from showing active content that could access your computer. Click here for options..."
if you click on the yellow bar you can select the option to "Allow Blocked Content".
If you allow the blocked content, you will then be presented with a dialog box, asking you if you are "sure that you want to let this file run active content". Click Yes to continue decrypting the file.
The second problem is a change that prevents some files from being executed on your hard disk.
Once you have saved the .html attachment, right-click on the file with your mouse and select "Properties" from the menu. In the dialog box that appears, click the button labeled "Unblock" and then Click "OK". You will then be able to launch the attachment successfully.
