if you have an external HDD you can copy the I Tunes folder across to your new machine and then import it into the new I Tunes. Be aware you can only have 5 machines with the same account name. If you were to exceed this number at some future date, you would need to de-register one of your machines. You can then sync your phone with the new installation.
It will warn you that it is already synced with another machine but just ignore it.