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Have you recently upgraded your operating system? If so, check to see whether you have a Windows.old folder, if there's nothing in there that you need you can delete it.
Check what's in your Recycle Bin and run Disk Cleanup.
You might also want to run a full anti-virus scan in Safe Mode using Malwarebytes : Malwarebytes Anti-Malware is a free download that removes viruses and malware from your computer
and by the sound of it a defragment of the drive wouldn't go amisss.