Are you using a mac?
It's most likely because your iPod was formatted on the Mac [thus using the HFS+ file system].
Windows can't read this [unless you use a program like MacDrive].
A better solution [and this is what I do] ..... go into VirtualPC (or do it from your friend's computer), and format your iPod using FAT32. Thus, it's read AND write capable on both Mac and Windows.
Downsides:
-your write speed may decrease just slightly on the Mac [not too big of a deal for me]
-you are limited to 2GB in size for any single file [this can be bothersome if you plan on importing around big compressed >2GB dmg files]
