How Can I Get The Music Off Of An Iphone To A Nw Computer

I recently got a new laptop because my desktop broke and i had most of my music backed up to a portable hard drive, but i have music that i purchased on my iphone that isnt there yet. When i go to sync the iphone it says that it is synced with another library and asks to erase it. I have tried programs that allow you to view the music but it only appears when you look from the iphone. Help?