If you want to make sure that it writes to the first partition (you may have noticed tha Linux does not use the C:/D: notation) you have to specify it manually. The best option is to start from the CD you made in Live mode (not install mode), then start GParted and remove your Windows partition. Be careful not to remove your data partition (your Windows partition should be much smaller than 700GB, I would guess). After that start the installer and when it comes to partitioning tell the installer to use the free space.
Having said that, installing an OS and changing partitions is a system critical task. You should make a backup of your valuable data before that. In fact, you should always have a backup of your valuable data, otherwise it can't be valuable for you.