XP pro has settings to automatically share all drives/partitions on your computer, without your knowledge, to see if any of your drives are being shared unknowingly on your network, do the following:
right-click "my computer"
go to: computer management -> system tools -> Shared Folders -> Shares
If there are any items listed which are of the form 'X$' these partitions are being shared privately. They are not visible to network users, but they are accessible non-the-less. Some computers will prompt for passwords protecting them, but there are simple flaws in networking which allow access around this.
Thus any partitions you do not want shared, right-click them under "my computer" and choose do not share as opposed to the setting which is chosen now.