my purchased songs won't transfer from itunes to iphone

Hello,

I have had this problem before and frustratingly can't remember what I did to fix it, I know I found the solution on these forums but can't find it! I've since had to get a new hard drive for my windows laptop so whatever was wrong has come back!

My laptop kaput meaning i lost my files including my music so I have a lot of music now on my phone that isn't on my itunes, meaning I don't want to sync and lose all the music on my phone as i don't have it anywhere else.

I re-installed itunes on my 'new' laptop and have downloaded some new music from itunes. When I plug in my iphone I can't for the life of me get it to transfer to my iphone.

I've tried 'sync only selected songs/videos' but that doesn't work and when i check 'manually manage music' it comes up with a message saying the iphone is synced with another library and everything on my phone will be deleted if i sync with this library.

I remember the solution when it happened with my last itunes was something to do with opening a new itunes and deleting my old one but i can't remember!

Would really appreciate help, this is so frustrating!!