You're trying to access your Windows 7 partition from booting in Windows 8? Do you have an external hard drive or anything which is not the partition that an OS (Windows 8 or Windows 7) is installed on? Try moving it to that location first, then try opening it in Windows 8.
I personally would suggest that you keep all personal files separate from your Windows installation partition... That's what I do, it allows for easier reformatting if you ever need to reinstall, because you don't have to worry about collecting all your personal data, it's already saved for you on an outside location, and you should be able to format no problem without affecting the partition or drive that your data is saved to.
There will be issues with trying to access other Windows install locations... You can change and fool around with the permissions but I wouldn't recommend it. Bypass the need to do that, and work around it in my opinion.
When you start fooling around with the data on your other Windows installation, there's no telling what kind of damage you CAN do if you do something which you shouldn't have done. Because that Windows installation (with it's files that you're trying to access), at the point you're booted into Windows 8, can practically be thought of as in "offline mode".