I just read your long paragraphe, I'm out of breath! Your life must be exciting and full of adventures!:-)
The changing of the background is more complex only with Windows 7, it has nothing to do with the computer brand.
As for computer quality,through my experience on this forum, there seems to be more techical problems with Acer than Gateway. I have an HP desktop. Just as an example, the other day, I purchased and ACER LED screen. Even with Acer's customer help I could get a good resolution. I returned it.
As for your background,
1. after entering the registry editor, do to edit and search.
2. Copy and paste:
3. Click okay
4. When the search is finished, look in the right pane for the value to change
5. Save the change and restart your machine.
Let me know if you succeed or not and we will see about another miracle.