If your pc is old and tired Windows 10 will more than likely make it worse not better
If you do decide 2 install Windows 10 you should not need to backup your photo's and documents but I would anyway just in case something goes wrong, and you should backup your photo's and documents to an external hard weekly or monthly depending on how much info is added / changes
Download CCleaner to use instead of Disk Cleaner: https://www.piriform.com/ccleaner/download
What are you doing for it 2 say files not found????