Acer Aspire 5742G - Where can I get a copy of Windows 7 Home Premium?

I recently installed Windows 10 on my Aspire 5742G only to find it not functioning correctly. Acer has since told me that my 5742G will not support Windows 10.
I am now unable to reinstall Windows 7 via Alt-F10 as it appears the installation of Windows 10 has corrupted the file(s) required. (A Windows 7 disc was not supplied with the purchase.)
I downloaded and installed a copy of Windows 7 Home Premium from Microsoft but it won't accept the product key supplied with my 5742G. I am now constantly reminded that I have installed a non-genuine copy of Windows 7. Consequently some MS updates are not available.
Where can I obtain a copy of Windows 7 Home Premium which will accept the Acer supplied product key?
Thanks.

Accepted answers

All comments

Gawain

when you say you installed 10, was this a totally clean install (from a downloaded 10 .iso) or a "free upgrade" within windows 7?

nstewj

From a downloaded Windows 10 .iso. The free upgrade deadline was well past.

Gawain

okay, if 7 is installed on the laptop now, can you go to the command prompt and type in SLUI 4 then press enter/return in the keyboard. This should take you to another page with a load of phone numbers etc on it that'll eventually take you to a MS operative - explain that you've reinstalled 7, used the product key on the underside of the laptop and it's not re-activating - hopefully they'll be able to activate for you (i know this is an OEM but i've used slui 4 before to activate a stubborn reinstall of both 7 and 10 oem, it's not an .iso issue but worth double checking you downloaded the right version for your machine - eg 32bit or 64bit etc). It is a freephone number, so just select the country you're in after the slui 4 command etc