Get a spare disk and do a clean Windows 8.1 install on it and bench mark it compared to the ages old install of Windows.
This problem hits OSX too, and always makes me lose my Smug Mac User grin. It's even worse in OSX because the filesystem, HFS+, is absolutely bonkers about cruft that builds up. HFS+ is worse than Panic at the Disco. HFS+ is almost worse than scabies.