By "checking" your drive do you mean you used a partition management application to look for the hidden boot and EFI partitions that were left behind by Windows 10, because I'm quite sure that when you installed Windows 10 it is very likely you didn't do it correctly by choosing the custom option and then deleting ALL of the existing partitions on the drive. Because if you had, there wouldn't BE anything for the system to ask you about in regard to Windows 10.
So, what you need to do is reinstall Windows 7, and follow THIS guide STEP BY STEP. Do not skip ANY steps thinking you know better than the guide. Every step is there for a specific reason.
https://www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
IF, IF, that is EXACTLY how you did it, and you DID delete ALL existing partitions on the drive, not just the C: partition (Yes, there are more windows OS related partitions on EVERY installation, 7, 8, 8.1, 10, than just the C: partition where the actual operating system files are stored), and not just "formatting" the C: partition, then it's likely you have another drive attached to the system that has those boot and EFI partitions on them due to having been used as a boot drive for Windows at some point in the past. If that's the case, you'll need to use a partition management application to find and remove those partitions on whichever drive they are on.
Furthermore, going from Windows 10 to windows 7 unless you are doing for reasons of not being able to use a legitimate activated version of Windows 10 because you don't actually have a qualifying product, is a really terrible idea. There are plenty of ways to bend Windows 10 to you will in terms of the desktop appearance, start menu, task bar and shell behavior while retaining the much better driver support, memory management, virtual memory management and other features of Windows 10 without stepping on your own, ahem, you know, just to do it.
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