MP3 files alone contain no metadata, but you can use ID3 tags within MP3 files to support metadata.
- Song Title (30 characters)
- Artist (30 characters)
- Album (30 characters)
- Year (4 characters)
- Comment (30 characters)
- Genre (1 byte, giving 256 possible Genre IDs)
ID3v1.1 Added a track number field to this.
ID3v2 adds many more fields, one of which is a user-defined text frame. This can be used for any data you want and could support a "user tags" field.
Why Windows 7 chooses not to use it, I don't know, but it doesn't matter really does it? There isn't anything you can do. What the client chooses to do with ID3 tags is up to it.