I Had To Reinstall My OS On My PC & Now I Can't Sync My iPod

I had to reinstall Vista on my PC. As of yet, I have been unable to get my music from my Seagate external hard drive. But I was able to send my purchased songs from my classic iPod to iTunes. Only, when I purchased a few songs tonight at iTunes, I was unable to send those songs to my iPod. I keep getting a message stating that my iPod is synced to another iTunes library--obviously the one I had before reinstalling Vista, and can't be synced with a seond iTunes library. It just keeps asking if I want to erase everything on my iPod and put on it only the songs currently in my iTunes libarray. That is of course something I do not want to do! How can I reset my iPod to accept the current iTunes library as the one it's suppose to sync with?