iPods only allow you to use the one copy of iTunes to sync your iPod. The error message you're seeing is to help ensure that you use just one copy of iTunes to manage your device.
Fortunately - there's a way around this. The steps listed below will copy the contents of your music files from your iPod to the computer of your choice. Note - these steps can be performed on any computer, it does _not_ need to be the PC that you use to sync your iPod.
___Step 1 - Enable View Hidden Files and Folders___
1) Open My Computer
2) Click the Tools menu option
3) Select "Folder Options"
4) Click the View tab
5) In the "Advanced Settings" section, select the option for "Show hiden files and folders"
6) Click OK
___Step 2 - Retrieve Your Music___
1) Connect your iPod (Note: If iTunes opens and asks if you want to sync the iPod, click Cancel, then close iTunes immediately.)
2) Open My Computer
3) Open the drive associated with your iPod
4) Open the iPod_Control folder
5) Copy all of these folders to your hard-drive
6) Open iTunes
7) Add the copied files to your iTunes library
Note: When iTunes copies the music to your iPod, it doesn't keep the file names you had on your computer. There are free applications (MP3 Tag Tools) that will allow you to rename all of the files based on the MP3 tag. Or, you can set the option in iTunes to organize your music library for you and specify the location of the copied folders. iTunes will then rename all of your files.
Additional Note: There are 3rd party applications that will do this for you, and even rename the files to represent the MP3 tag information. Wide Angle Software makes an excellent application - iPod Copy - that you can purchase for $19.99 at http://www.wideanglesoftware.com if you'd rather not do this manually.