Actually I found an easier way to fix the file permissions problem. Rather than assigning myself to the folder under security, I simply modified the security for the "Home user" which already exists.
This is how you do it.
- In file explorer, right click on the folder and click Properties
- Click on the Security tab
- Click Edit
- Select "Home Users (yourcomputer\HomeUsers)"
- Check the "Allow" checkbox next to "Modify"
- Click OK
Note when I say click on "the folder" I mean the root folder below which you need to have access. For most people this is everything below C:\Windows\Users\YourUserName, but you could also do the same on pretty much any folder. I recommend you do this on your username folder under C:\Windows\Users. That will include your documents, pictures, music, etc.
Depending on how much stuff you have on your PC it could take a few minutes to propagate the security change to all the files in your folder.