Can you elaborate on 'transfer existing account'? Also is it an old old old iPod or iPod Touch with Wifi? I'm assuming it's an iPod Touch with Wifi. Your iPod has no 'account' per se, just data. Unless it's an iPod Touch with Wifi that you've used to purchase content from iTunes. Which, would mean you've used an Apple online account (iTunes, Me, Mac, iCloud, etc...) that consists of an email address (username) and password. That online account is omnipresent. You can log into it from any device really as long as it has internet. Open a web browser and go to www.icloud.com. Log into your Apple account.
UNLESS....you're referring to your music, pics, etc... on the iPod itself.
PICS: You can connect the USB charging/data cable from the iPod to the computer. It should auto install the necessary drivers for the iPod. You can then 'import' your images. Music and other data is another matter. You may need to install iTunes for the drivers.
MUSIC/DATA: It's best if you enable iCloud backup on the iPod/Touch. It 'should' back up all your purchased stuff, including app data, to your personal iCloud storage space, provided that you purchased enough storage space. 5GB is free. Your mileage may vary. It will take a while (weeks) to do one complete full backup to iCloud depending on how much data you have. You can purchase 3rd party software to transfer music from the iPod/Touch to the computer. Apple has a strict DRM policy to not allow that to happen. But 3rd party software is your best bet. Google is your friend.
EDIT: Apple keeps track of all your purchases. You can re-download any music/apps free of charge as long as you use that same Apple Account in iTunes. You can download them on your Win7 machine.