how do i get songs i purchased 8 years ago back?

Ever since I joined Itunes i have purchased over 400 songs. I have been using new computers, which have made me download a new Itunes to various computers, but my songs would not come along. On each computer I would only have the songs that I purchased with that computer only. And I would like to have all my previous songs back. I payed for them and would like to enjoy them again.