Are you using Vista? If so, then yes, you should be able to edit the ID tag by right clicking the file, clicking Properties, then click the Details tab. Then you can click on any line and enter the info.
If Windows is not allowing you to do that, the main reason is that you don't have permission. Is the file in your user's Music folder? If not, try moving it to that folder because you should have permission to do anything you want there. If you can't move the file, you may need to be logged in as administrator. If it's still not working, try turning off the UAC and see if you can do it. But you're right, you should be able to edit the tag from Windows Explorer. I just did it myself.