I am really grateful for your efforts on my behalf. But this is not proving to be easy:
1. NET.4 download completes, but when I click to install there is an alert saying "some or all of NET.4" is already installed on the computer and so the installation will not proceed.
2. The attempt to download the AMD driver repeatedly fails. I have made four attempts to download it, and each time the download just hangs and does not complete.
3. Just to see what happens, despite the failures noted above, I followed the directions in the article you linked to from  above. The "Programs and Features" menu (of course) does not list "AMD driver". It does list ATI Catalyst Install Manager, but I assume this is not the right software for current purposes.
4. Clicking CCC from the desktop menu still has no effect.
I really do want to deal with this issue. As things stand my new minitor is smaller than the old one I replaced, and although the quality is good, the size is a permanent irritation. While I realise that the issues originate in the PC and Windows7 side of things, this is an issue that will (I assume) affect all Dell customers using HDMI, and I see that people have raised it in relation to other Dell monitors. Is is not possible for Dell to provide a single "fix"?
Anyway, I would still be very grateful indeed if you could review my efforts to follow your suggestions, and advise me of some alternative or additional steps I can take to achieve the objective you think will solve the issue in my own case.