Just getting back in town, was gone last week. Thanks for the info on your laptop. It sounds like it did pick up an additional User ID; like I said you added it at some point in your original laptop Win7 install that came from the factory. Many folks add Users temporarily and then never use them again. However, in Win7, if you had added an additional User it would show up on the Windows Logon screen when you first turn the laptop on. This is most likely where Win10 picked it up from. Many of my Customers add a friend or a family member to their Win7 setup, if they are staying temporarily in the household, or loan the entire laptop to a friend of family member. What happens there, is that Customers will change the Security setting from "STANDARD" to "ADMINISTRATOR", and the person who got your laptop on loan or was using it may have tried to delete your Account (who knows why!), and scrambled your User Profile.
If this didn't happen then the next most likely cause was that YOU added a User temporarily, from visiting friend or family member staying with you or whom you loaned the laptop to, and when they left your home or returned the laptop to you, you then attempted to delete that User name or User Account and may have forgotten about it. I mentioned this above, but you didn't respond about that so I am left to guess. But, I've seen this done many times before.
So you know, your attempts at removing the GUEST and ADMINISTRATOR accounts are not possible, as those are default accounts that are built into Windows (all versions) and can never be deleted; even with your Owner or Administrator permissions!
If the case that you put in that additional account into Windows or a friend/family member did so, and it's stuck there, and Win10 picked it up--I agree with your diagnosis that the best way to get rid of it is to reinstall your Win7 from factory Recovery Discs or Recovery Partition on the hard drive. Prior to doing this, I urge you to BACKUP ANY AND ALL PERSONAL DATA TO EXTERNAL MEDIA SUCH AS DOCUMENTS, PHOTOS, MUSIC, VIDEOS, AND EMAILS!!!
Upon doing this you will only have 2 User accounts (the Windows defaults); your signon name (say KATHRYN) which will automatically be an Administrator account and the Guest account mentioned above. Neither of these User accounts can ever be deleted. When you get the Win10 gadget after your reinstall, which may take several days to show up again, run the upgrade, and you should only have those 2 accounts in your Win10. This most likely will fix your problem.
So you know, there is a way to repair your corrupted User Profile, but it hasn't been tested in Win10 yet, but did work on Win7/8/8.1, and that involves copying over all the info in your existing User Profile to a new copy and then attempting repairs with tools like CCleaner or Registry Mechanic. However, I don't recommend these tools for novices--they require expert User only. If you wanted to, you could take your laptop to your local Computer Pro, and if he's competent with these tools he might be able to fix your existing User Profile in Win10. No guarantees however, and it's going to cost you money. I recommend if you have more time than money available to go with the already discussed Win7 reinstall-->Win10 upgrade method which you can probably do yourself.
Let us know how it turns out.