okay im not sure what exactly you asked.
First is this music you purchased? If so I'm pretty sure you can download it all again on your itunes.
Now what I think your problem is you have an old computer with a lot of music, and a new laptop with only some of your music.
Well if you have a USB flash drive what I would do is boot up the old computer and put all the music into a folder, and then put the folder on the usb drive then put the new computer.
When you go into itunes you just have to go to file>add folder> and select the folder and then it will add your music
go to start>my music>itunes> and rename the library.xml to anything and you it will automatically remove all music from your library so you do not have to manually get rid of repeated songs etc.
I found this link
You must login or register to view this content. but this seems pretty technical for something really easy with a usb drive or external hdd.
you could probably also use open ssh on a ipod touch/iphone to do this, but that would require jailbreaking