I download files such as wmv, avi, mpg etc onto the desktop and then burn them to a standard DVD-R disk for later use.

Recently something changed and when I load a burned disk, where I used to be able to drag a file from that disk which would then copy to desktop, now I get a white circle with a bar through it when I attempt to do this, and cannot copy files to desktop from any burned disk.

I don't know what has changed or how to fix this problem.

Why has this changed and what can I do about it?