I've had the same issue on and off for the past few months and I'll try to provide as much info as possible in the hopes of finding a solution.
On my XP machine, I have a shared folder in which I'm constantly adding and removing files. Most of the time, I can access this shared folder from my iBook, and I can always access it from my other XP desktops and laptops.
However, once in a while, usually after adding some new files to the share folder, I cannot copy the new files from the XP desktop to the iBook. I don't think it's an admin privileges issue as funkychunkz stated above because I can still access the older files in the very same shared folder. In fact, it seems that I can access all files that were put into that folder before a certain date; after that date, no new files can be copied to the iBook. The interesting thing is that I can create a new shared folder or move my files to a different shared folder on the XP desktop and the files become accessible from my iBook (although I have found that this does not always work).
I've tried playing around with privileges and share settings on the XP machine, but to no avail (rebooting the machines have no effect either). The last time this issue occurred, it just went away after a few weeks, but it just returned last night and I'm stuck with the same error message again: "The operation cannot be completed because you do not have sufficient privileges for some of the items."
Unfortunately, this quirk doesn't allow me to open the files from their current location either, so I'm stuck with having to connect to the iBook from my XP machine to move the files. If I discover I forgot a file or need another one once I'm using the iBook, I have to trot across the house and move it from XP.
