Can't say but it shouldn't be because Vista has never been supported by LR6, make sure that your version of Windows 7 is 64bit and not 32bit.
You can also download a copy to install from ... Install Photoshop Lightroom
You would have to cancel the autostart of the dvd, and then use Windows Explorer to locate the installer file on the dvd, and right click it.
I'm guessing that the file is called setup.exe.