Disk Cleaner: System

In this part of Disk Cleaner settings, you can specify the information generated by the operating system, which should be removed.

Activity History (Timeline)
OS: 10
Risk Level: YELLOW

CHKDSK, ScanDisk Files
When Chkdsk or ScanDisk checks a disk for errors, Chkdsk might save lost file fragments as files in the root folder on the disk. These files are unnecessary. Often these can be deleted as the information is elsewhere on your hard disk.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: YELLOW

Crypto Cache
OS: 10, 8, 7, Vista, XP
Risk Level: GREEN

Downloaded Installations
The Downloaded Installations folder is where some programs will install their setup files. This is in case you want to modify or reinstall the program. If you're not going to do this, these files usually can be removed.
OS: 7, Vista, XP
Risk Level: YELLOW

Downloaded Program Files
Downloaded program files are ActiveX controls and Java programs that are downloaded automatically from the Internet when you view certain pages. These files are temporarily stored in the Downloaded Program Files folder on the hard disk. In most cases, these files can be deleted.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Files Discarded by Windows Upgrade
When you upgrade Windows just in case, keep a copy of all files that have not been moved to a new version of Windows and have not been identified as system files of Windows. If any personal files were not lost as a result of the upgrade, you can delete these files.
OS: 7, Vista
Risk Level: YELLOW

Hotfix Uninstallers
Those are the uninstallers associated with every Windows security and stability patch (hotfixes) you've installed. While hotfixes correct known issues with the operating system, hotfixes can at times create unforeseen conflicts elsewhere. In some cases, resolving the issue originally addressed by a hotfix may be less important than avoiding the conflict created by applying the hotfix. For those situations, hotfix uninstallers allow users to undo the changes the hotfix made.
If you delete those, you lose the ability to uninstall those hotfixes. After you installed a hotfix, wait for some time to make sure no problems come up...if none do, you can to remove the hotfix uninstallers to save space.
OS: XP
Risk Level: YELLOW

Installer Cache
OS: 11, 10, 8, 7, Vista, XP
Risk Level: RED

Memory Dumps
If you happen to crash the system with BSOD (Blue Screen of Death), then at the same operating system retains some memory to disk for analysis of failure by specialists. If you're not going to analyze this information, you can remove it.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Prefetch Directory
Windows contains a directory known as Prefetch in which it stores files that are used to start Windows and other applications. By prefetching, Windows can read ahead into the cache the data that is expected to be requested (remember it’s much faster to read data from the cache). You can see overall how this prefetching can improve performance.
Over time some of the files within the Prefetch directory are no longer used. The result is that it can noticeably slow down your computer’s boot time.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Recent Document History
Recent Document History contains links to files you’re opened, for quick access to recently opened files.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Recycle Bin
The Recycle Bin contains files that you have deleted from the computer. These files are not permanently removed until you empty the Recycle Bin.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Search
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Temporary Files
Programs sometimes store temporary information in a Temp folder. Before a program quits, the program usually deletes this information. In most cases, these files can be deleted.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Thumbnail Cache
On Microsoft Windows operating systems (starting with Windows XP), a thumbnail cache is used to store thumbnail images for Windows Explorer's thumbnail view. This speeds up the display of images as the smaller images do not need to be recalculated every time the user views the folder. This effect is more clearly seen when accessing a DVD containing thousands of photos without the thumbnail cache files and setting the view to show thumbnails next to the filenames. The problem is that over time these files are becoming more and more. Therefore, they can be periodically removed. All the necessary files in the future Windows will create anew.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Windows Delivery Optimization Files
OS: 11, 10
Risk Level: GREEN

Windows Diagnostics Infrastructure Logs
OS: 11, 10, 8, 7, Vista
Risk Level: GREEN

Windows Error Reporting

Windows Font Cache
OS: 11, 10, 8, 7, Vista
Risk Level: GREEN

Windows Log Files
Windows log files are generated by the system to help with troubleshooting any potential errors, as they give detailed reports of everything the system is encountering, be it security issues or something else. If you're not going to analyze this information, you can remove it.
OS: 11, 10, 8, 7, Vista, XP
Risk Level: GREEN

Windows System Assessment Tool (WinSAT)
OS: 11, 10, 8, 7, Vista
Risk Level: GREEN