iTunes and your computer obivously think that files are being used. This could be anti-virus software or it might be any programme that you were using to modify files that are on your iPod.
For example, if you were using a photo editing programme to modify a photo that's on your iPod, perhaps the programme hasn't "let go" of the photo.
A solution is to shut down the computer. Once the PC is off, everything is not running and it is safe to disconnect the iPod.