I wonder if its possible to pass a already existing windows install into qemu (fedora 40 kde)

Yes. Yes it is. I had a single Windows install that was my main system. I eventually decided to semi-switch to Linux, and passed that SSD through to a VM, along with the GPU and half of my USB ports in order to continue running that Windows install for gaming and Adobe stuff. You just have to keep messing around with the config of the VM, and look up info about getting your UUID from Windows to add to your VM config, so your Windows license/activation will keep working as is. There are guides and info for all of this if you Google around. Don't remember exactly where I found those guides, as I ran that setup for about year and then switched that Windows install back to bare metal, and it has been about another year since then. I do remember that I got the same boot issues as you when switching back and forth between bare metal and VM, and I had to run the Windows boot repair tools to get everything working. Trying looking into that.

Possible. Disable bitlocker if you have it enabled, create a recovery disc from Windows, pass through your recovery disc and SSD to QEMU, boot from the recovery disc, run automatic startup repair. Ensure you use the correct firmware type in QEMU: If your SSD is GPT then use UEFI. If it is MBR then use BIOS.

Managed to fix it (By reparing it in hirens bootcd) but gives me a system thread excepsion not handled bsod Fixed this issue by changing the cpu to qemu64

Are you only using QEMU, or are you using some sort of wrapper around it? QEMU is quite advanced, if you aren't already, I'd recommend you use some sort of GUI like virt-manager or something.

Can you share your config?

Does it BSOD or just reboot after the Windows logo?

You might have to pass the drives through as IDE, Windows might not have the proper drivers for anything else. Once you can get it booting you can mount a blank drive as virtio, install the virtio drivers, and then change the OS drive to virtio.

Is your qemu using bios or uefi? You cant switch between them without something like that happening.

Hit a key.

It will give you options. One of them is generally continue boot.