Solution:
In Office 2003, you might have noticed a printer called Microsoft Office Document Image Writer. If you print a file to this virtual printer, you'll get a file with an extension .MDI, which stands for Microsoft Document Imaging. This format is Microsoft's proprietary file format that's useful for storing raster images of scanned files.

You're getting this error because by default, Office 2007 setup doesn't install Microsoft Office Document Image Writer. You can drag and drop the .MDI file into OneNote to read it, but if you don't have OneNote installed then you can simply install Microsoft Office Document Image Writer.

Here are the steps:

1. Go to Control Panel in Windows Vista.
2. Under Programs, click Uninstall a program.
3. Highlight your Office 2007 version (e.g., Microsoft Office Ultimate 2007).
4. Click Change near the top of the window.
5. By default, Add or Remove Features radio button is highlighted. Click Continue.
6. Go to the Office Tools section and highlight Microsoft Office Document Imaging.
7. From the drop-down box, select Run All from My Computer and click Continue.
8. Click Close when finished.

This procedure will add a new virtual printer called Microsoft Office Document Image Writer. Now when you double-click a file with an .MDI extension, you'll be able to view the file without getting any errors.