Hello.
I have tried to install Proxmox and windows in dual boot but it's tricky.
Dual boot without installing Proxmox over Debian it's impossible I think. Proxmox will be recognized by the bios as a boot option but not displayed like in Linux and windows dual boot options. And also, you can't have 2 OS on the same drive if you install directly Proxmox.
So there are 2 ways:
1. If you have one disk in PC and you want 2 OS's on the same drive, install Debian and Windows, install Proxmox on Debian and there you are.
2. If you have 2 drives for OS, on one use Windows and on one install Proxmox but in this case use F12 to select OS.
I have the second configuration and works perfect.
Until now this is all I managed do. Cheers!