Windows 10's C drive is filling up fast, with system files taking up nearly 90GB

That 90 GB may include Windows updates, the component store, restore points, the page file, and hibernation, so identify the category instead of deleting folders. Open Settings > System > Storage and inspect System & reserved, Temporary files, Apps, and Other. In Temporary files or Cleanup recommendations, review each item and remove only what you recognize; keep Downloads and Previous Windows installation if you need those files or rollback capability. Next, run Disk Cleanup, select C:, choose Clean up system files, and review Windows Update Cleanup and temporary installation files. Turn on Storage Sense for cleanup. Do not manually delete Windows, WinSxS, pagefile.sys, or hiberfil.sys. Search for Create a restore point, select Configure, and check System Protection usage before changing its limit. After Windows Update and a restart, recheck Storage. If it still reports about 90 GB, share the Storage category breakdown so the specific consumer can be diagnosed safely.