Itunes delete songs if i transfer them

Hello,
i recently bought a new computer and downloaded my itunes. before on my old computer i had an itunes with like 200 songs. since i got my new computer i bought 20 songs and tried to transfer them onto my ipod. Everytime i try to however, a message pops up saying that if i transfer the new songs onto my ipod all of my old songs will be deleted. what should i do?

Extra:
i also get a pop up saying that a newer version of itunes is ready for download. Should i download it or will it just make things worse? there is also a pop up that says this "Itunes has detected that it is not the default player for audio files. would you like to the default programs control panel to fix this?"