Why Do All My DVD Say that They are Written (Burned) on?

I Have a Mac who I am Bootcamping to Windows XP Professional. All the DVDS are new and the Mac is almost two years old with a new hard drive, I have tested the DVDS and they are blank in other computers, yet when I put them in the MAC (windows Side) they say they are already burned, thus I cant write in them. [I'm just curious~ Sooner or later im going to format it to windows 7 but i still would like to know. Thanks ^ ^] -,-" no MACS SUCKS< post -.- pfft.. MY O.S is Bi, I like both Windows and Mac