I upgraded to Windows 10 before Microsoft notified me to do so. I did not have a Product Key. I was able to boot the computer, but I had many problems. Windows 10 did activate. This allowed me to erase Windows and do a clean install. This resulted in an installation where the earlier problems did not reappear.
When I installed previous versions of Windows, I did not allow updates until I had installed the Boot Camp Support Software (BCSS). Windows 10 started downloading updates early, before I was able to install the BCSS. Some updates were drivers that failed to properly install.
I easiest way to fix this problem was to deny the computer access to the internet until I could install the BCSS. If you wait for the Windows installation to complete, you will have to create a local (offline) account. Once the BCSS is installed, you can connect to the internet and switch to a Microsoft (online) account. The window where you would make the switch is shown below.
The other option is to switch to audit mode during the Windows installation. This will allow the BCSS to be installed before you create your account. You and then enable access to the internet and complete the Windows install by directly creating a Microsoft account.
The BCSS for your Mac can be found at Boot Camp Support Software 4.0.4033. These drivers are for a 32 bit version of Windows 7. I assume you are running the 32 bit version of Window 7 and desire to upgrade to the 32 version of Windows 10. If not, let me know. You may be able to upgrade to a 64 bit version of Windows 10 for free.
If you do a clean install, you will probably need to run the BCSS in Compatibility mode. Right click on the icon next to the "setup" application and select properties. Next, click on the Compatibility tab. Mark the check box for "Run this program in compatibility mode for: Windows 7". Finally, click the "OK" button.
Also, note that if you have Windows 10 Pro or better, you can disable automatic downloading and installing of updates. You can also review and selectively disable (hide) updates.