ITunes authorization help?

Okay so we had a copy of windows xp, and on that I had my iTunes library with purchased songs. We got a new xp operating system, but didn't wipe the computer clean, so it still has everything. Now when I go to listen to purchased music, it says I need to authorize the computer to the address that bought it (which is mine so I thought no problem.) So i authorize it and it says authorization successful etc. Then when I listen to the song the same authorization screen pops up. it says it works but it doesn't. It's like it thinks is a different computer so it makes me re-authorize, but knows its the same computer so doesn't actually change it. Either way I really need help because much of my library is now off limits.