Hello Ralf Richii,
Please give your hardware specs in your signature.
Make sure which one is your debian and windows and other partitions that may have data on it by booting from a rescue disk.
See what the dump of this is:
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lsblk
The above command will show you the disks and all the partitions each disk has.
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blkid
The above command shows you the attribute of every single partition on your system irrespective whether its mounted or not eg: whether its a windows partition i.e. ntfs or some linux i.e. ext4.
By recognising which partition is where and on what disk you will know which partition you want to format or recover or install grub too.
If you want to format the debian install go ahead(backup any useful data). Install the grub on the same disk, the windows loader will get auto picked up by grub.
NOTE: Run the above commands from your rescue linux disk so that the partitions are correctly recognised by them accordingly.
No worries, Debian is way better OS than ubuntu but the problem is its mainly
targeted for servers with multiple processors, NIC cards and RAID cards and with support out of the box.