REFORMAT IT!!

I hate it..
I wanted to reformat this computer..
The space is low..
Sister stopped using this computer since she's in college..
She installed and downloaded a lot of things into this computer..
Ahh.. Hate it!!
The space is low and it's lag~
All the things are quite messy! TT
Asked daddy to reformat it..
But he told me he want to reformat Window 7..
Haiz.. We don't have Window 7..
Asked him if buy how much?
He told me :"I don't know~ Never buy b4"
Urgh.. TT HELP!!