Oh, it's a pain alright.
The power button and circuit board is one unit. Dell has not published a guide to remove the front panel and control panel of the Dimension 8400, but you can use the guide, <ADMIN NOTE: Broken link has been removed from this post by Dell>
here, it's for an Optiplex GX50 Small Mini Tower, but the procedures are the same.
Please don't send me questions about your system by DCF Messenger.
Post the issue in the appropriate Board, where they will be answered.