eLPuSHeR said:
You can reclaim almost 10GB+- using the compact command-line util. Open an elevated cmd prompt for invoking it.

E.G. compact /compactos:always
compact /c /a /i /s /exe:xpress16k "c:\program files"\*
compact /c /a /i /s /exe:xpress16k "c:\program files (x86)"\*
compact /c /a /i /s /exe:xpress16k "c:\windows\assembly"\*
compact /c /a /i /s /exe:xpress16k "c:\windows\inf"\*
compact /c /a /i /s /exe:xpress16k "c:\windows\InfusedApps"\*
compact /c /a /i /s /exe:xpress16k "c:\windows\fonts"\*

Do not touch anything else. Your system might not boot or something.

PS. LZX is an even stronger compression method than xpress16 (exe:lzx) but it may need some serious horsepower cpu.
What happens if you compress files from the AppData folders though?

No boot?