How do i sync my old iphone to new windows laptop which is authorized without losing all the data on the iphone and having to sync only with what is on the laptop??

I have a new laptop and i have authorized it with my apple ID but I can't sync my iphone with it because it is currently synced to another computer. Every time I try and sync my music it comes up with the message ' are you sure you want to remove existing music, films, tv programmes,books and tones from this iphone and sync with this itunes library' which i DONOT want to do! Also on my laptop none of my apps are there so I would assume i would lose all of those from my Iphone as well!

Please help, looked at other threads and all they suggest is to click on 'transfer purchases' but this is not an option anywhere on itunes..!

Any help would be greatly appreciated!!