setting up home sharing

I'm really struggling to get my home sharing set up. I originally had a dell laptop (windows) for many years, and have recently bought a macbook air. My previous purchases and old media library are all on my old dell laptop. They are both connected to the internet via my home router and can get on the internet individually just fine. They both have the latest version of iTunes, and have home sharing turned on. Each time I try to import music to my mac, and select my dell laptop (which it sees just fine), it asks me for a username and password. The same happens in the reverse direction when I try to get media added from the mac to my dell laptop. I have no idea how to find the password to get them both to work together, or unsure whether the username is correct either? Please can someone help? I've tried to navigate through apples online help, but it mentions nothing about passwords? Any ideas anyone?