Can I delete these leftover files? How do I delete hidden files?

CCleaner, if you want, there's an advanced option called Wipe NTFS free space, it will take a few hours *depending on the condition of the clean and how secure it is* but It'll get it done. Yea, just don't delete your C:\Windows folder, or anything in it. Program files and program (x86) if you ahve it are probably not things you want to mess with, either. If you uninstalled a program, alot of times it doesn't delete the folder it was in. To find hidden files, it's in folder options somewhere, too lazy to research it, but you can google it.