If you use the NTFS filesystem, you may notice that you may not be able to access certain files or folders after you have reinstalled Windows. This is due to a change in ownership of the files or folders. To gain access again, open the command line with administrator permissions and run the following commands. Note, the takeown command with the /r flag will recursively reassign ownership of all files/folders under the current folder to the current owner.
cd <parent folder>
takeown /r /f .