The disk is either in an ext2 or ext3 file structure - both of which are native to linux - you should be able to read them in Windows if you use an installable file system driver - ext2ifs is the one I used, but it may not work with Windows7, there are others - google will help find them.
The best way to deal with your situation however - is to back up the data before the NAS fails - what if the failure takes the disks with it?