This happens a lot with Mac/PC users - music is downloaded with one operating system, but then the user switches to another operating system, but the process should be the same for Vista and XP.
Repair the iPod
When you connect to iTunes, there is an option to repair the iPod. This is actually a bit misleading, and instead of repair, you are actually erasing everything on the device and starting over with the factory settings. Make sure that anything you want to save is backed up first, before you try any kind of repair. If this is a hand-me-down iPod and you don't care about what is currently on it, go ahead and wipe it clean and start over.
Install the latest version of iTunes on the computer you are going to be using. Connect the iPod to the PC - hopefully iTunes will recognize the iPod. On the first page, where you see information about the iPod, you will be able to click on the repair button and start the repair process to bring it back to factory settings.
Authorize the Computer
If you are using a new computer and want to synch your iPod to this new computer, you can go to the iTunes app on the iPod while it is connected to the computer and deauthorize it. First, make sure that your files are backed up to an external drive or CD ROM(s). In the new operating system, download iTunes, setup the account and then import the files from the back up media. You can then go back to the iTunes store and "Authorize" the new computer.
In order to backup your iPod to a CD or external drive, go to Backup, then File and select Backup to Disc. You can also manage your music and video files from a third-party software system such as ephPod, which is a Windows application that is able to quickly transfer songs and libraries.