HD is almost full & PC run very slow but why?

Hi friends My laptop has become extremely slow and the HDD is almost full but I can't find what's taking all the space? The laptop has 6 user accounts and as far as I'm aware I'm the only one using it for iTunes.iPhone. All files are saved in the 'Shared' folder "I think!" I am ok on a computer and can easily folow any help and advice given here Many thanks Jon many thanks

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