There are two things that could be going on if this happens:
1) Some email programs will display multi-part messages as garbled code or an attachment if an email message doesn’t contain both HTML and plain text content.
To avoid this add either plain text or HTML content to your campaign if one or the other is missing.
Use the PREVIEW feature and send your test emails to different email clients (Yahoo!, Gmail, AOL Hotmail etc) to make sure your code is intact.
2) Anti-virus programs will sometimes “break” multi-part email templates up and cause them to display as raw HTML code.
This is not a common issue, but to fix this problem you need to change your anti-virus setting so that it accepts multi-part email messages. If you need more guidance, contact your anti-virus software company directly.