Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
this is an excellent tool for figuring out where storage is being consumed.https://windirstat.net/Be VERY careful when deleting stuff on c: (especially in the windows directory) unless you know exactly what it is.
Operating system drive you probably have a very large page file and/or other system Reserve space that matches your physical memory.
I went through this a few months back with a now-seemingly-wimpy 128 solid state C drive. There are plenty of sites with tips on what to clean and how, but I seem to remember dumping temporary files and restore points, and emptying the recycle folder helped me.The built-in Disk Clean Up in System tools made it pretty easy and also showed what you could expect to gain before ever starting.
Glad you got it figured out Dan....BUT, a 136 GB C: partition is pretty small. Especially when you have all that available space elsewhere.Not sure what the specs are of your drivesare, but a nice 250 GB or 500 GB SSD as your boot/C: drive would make your system come to life.