Access Denied when files is okay

I use Windows 7.  A couple of weeks ago, all my PDF's were readable.  Today, for example, they are readable in folders with a name beginning with a letter from 'A' to 'F'.  Then those that are in folders with a name beginning with a letter 'G' to 'Z' are not readable.  If I transfer all my PDF's to a memory stick they are all readable!  I've re-install the software several times.  Anyone with advice?