I know this has nothing to do with messenger, but try this:
To work around this issue, save the .xml file attachment to another location on your hard disk, and then open the file from this location. To do this, follow these steps: 1. Start Outlook or Outlook Express, and then open the message that contains the .xml file attachment that you want to open.
2. Right-click the attachment, and then click Save As.
3. In the Save Attachment dialog box, specify a name and a location in which to save the file, and then click Save.
4. Close the message window.
5. Start Windows Explorer, and then locate the .xml file that you saved in step 3.
6. Double-click the file to open it.