Jo det går att göra. Om det går snett så riskerar du att inte kunna boota, så var förberedd på det (backup etc).
här är en instruktion (för W7, kör du senare OS kanske du ska googla upp en annan guide för säkerhets skull, men borde vara samma eller liknande för W8.1/10):
https://windowsforum.com/threads/how-to-move-windows-7-boot-m...
De centrala delarna:
"First, make the boot partition active. In your case this would be C: or the Win 7 partition. Use Disk Management for this.
Then you need to make that partition the first active partition encountered during the boot. Set that drive as primary or first in the bios order. If you use the bcdboot command shown later, set the order during the reboot after adding the boot files. If you use the Repair Install Method, it must be done prior to that action.
Then you need to add the boot files to the C: partition (in your case). You can do this in one of two ways.
The way I prefer is to open an administrative command prompt. Do this by typing CMD in the Start Menu search box and hit CTRL+Shift Enter Then type the following.
bcdboot C:\Windows /s C:
Hit enter and wait for it to finish, then close the command window.
You can also add the boot files by running a Startup Repair 2 or 3 times to replace the original boot files.
"
