Jack Douglas wants a way to add comments or descriptions his files.
What you're referring to is called metadata--data that describes the contents of a file (as opposed to plain old regular data, which is the contents of the file). Different file formats have different metadata fields. You can see and edit a file's metadata by right-clicking the file, selecting Properties, and clicking the Details tab.
If there is no Details tab, that file format doesn't support metadata.
A number of popular file formats contain a Comments field in their metadata. These include.jpg, .doc, .docx, .xls, and .xlsx. When you go to the Details tab in any of these files, you'll find a Comments field that you can read and edit.
When you click on a file in the Vista and Windows 7 versions of Windows Explorer (but not in XP), some of the metadata is displayed at the bottom of the Explorer window. Unfortunately, Explorer doesn't display the comments field there. For that, you'll have to right-click the file, select Properties, and click the Details tab to read it.