Windows Shadowstorage is used to store some files, which can usually be removed savely. By default, the maximum size of this storage is unlimited. In case you need/want to save disk space, open cmd.exe in administrator mode and do something like:
vssadmin.exe list shadowstorage
vssadmin.exe resize shadowstorage /on=C: /for=C: /Maxsize=2GB
This limits the storage used for drive C: to 2GB.