Unknown error on my iTunes!

I'm trying to update (manually) my 30GB 5Gen iPod with my iTunes (ran on iBook G4) anyway, it's saying this
"Attempting to copy to the disk 'Sara's iPod' failed. An unknown error occurred (-48)."

and
"The iPod "Sara's iPod" cannot be updated. An unknown error occurred (-48).

Can anyone tell me if this is to do with my iPod or iTunes and what do I need to do to fix it because I haven't been able to update any new songs I have.
I run my boyfriend's iPod off my laptop aswell and this message comes up when I try to update his. So it can't just be my iPod.
Sometimes, i'm lucky and a few songs will go on. I just uploaded this particular album (from bought c.d) a few minutes ago so the files couldn't be corrupted. Someone help me Please!!

Find and delete the file:

com.apple.itunes.plist

Afterwards you may have to reauthorize your computer for your iTunes account.

As always, make sure your music and video is backed up somewhere just in case.

ok i found and deleted this file "com.apple.itunes.plist"
i tried to update again and the same warning appeared.
i dont actually have an iTunes account so i don't know how to authorise it again?