If your data were really important and it must be recovered at any cost, the best you could do is search your location for some skillfull guy who could help you to restore your data. Note, the more you use the drive right now, the less chance to save data.
And yes, recovery should be carried out on other computer. You have to have another computer to attach your drive there.
If your data wasn't vital, just forget about it. Do not despair, almost all of us went this way in past
If you still want to have both Windows and Linux at your box, some notes:
Do not create too large partitions for you systems. It is more than enough 100-150 Gb for Windows and 20-30 Gb for Debian. All the rest drive space should be issued as a seperate storage partition (to save your data there). This partition must be ntfs formatted in order to be accessible from both systems (Windows and Debian). And optionally, you could create little swap partition.
Do not use Debian installer or Windows installer partitioning tools. There is GParted liveCD with intuitive Gparted tool for partitioning. Make partitioning and start with Windows installation. Once it will be installed install Debian.