You could do a full clean install onto your pathetically small C drive, in which case it will delete everything on that drive first. So transfer ALL your data onto say your external drive, copy the W10/11 iso onto a USB stick, and start over clean.
You would have to move the locations of your Windows library folders (Docs, Music, Videos, Downloads etc) to the external drive.
Does your PC/laptop whatever it is, not have a slot for a 2.5 inch SSD?