On the disk clean-up, there are several items and I have no idea what is safe to delete and what should not be touched. For example, delivery optimization files.
There are utilities within Windows that can reduce the number of unneeded files (e.g. Accessories -> System Tools -> Disk Cleanup), and third-party utilities that can do even more.
I won't offer specific advice on what files to delete on someone else's system. Ya never know what lurks.
Do a full backup before running any such programs. And run a "verify" pass on it. If you're like me, do two full backups, in case the first one has problems down the line.