Outlook 2003 incoming email HTML code being received?

For some reason certain emails are coming in all messed up. Some are coming in with just HTML code when it should be a pretty HTML email and others are being received with a bunch of header garbage in the email. Along with this garbage, some emails that should have an attachment no longer have an attachment. Now, for some of these emails, when they're forwarded, the recipient receives the email how it should look. I can't figure out how to get this to stop happening. Here are some of the facts: she's running Outlook 2003; anti-virus is ESET NOD32 Business; in Outlook's email options "read all mail in plain text" is not checked; she's using Microsoft word 2003 as the mail editor. There are a few other things I've tried that I found researching this issue on the Internet, but I don't remember all of them. Any help would be greatly appreciated.