If you want to hide the partitions from each other, go into Computer Manager (Right Click My Computer -> Manage) and then select the Disk Manager. In there, right-click the drive that you want to be hidden and change the drive letter, in this window remove the letter that is currently assigned. If you are dual booting, remember that you have to leave the 100MB shared disk alone as that holds the partition information.
If you are looking to only allow read access to the other drive, you can do that by right clicking the drive in My Computer, going to the security tab, click advanced and make sure inherit is unchecked (it shouldn't be at this point). From there, delete all the accounts in the list from the menu and then add an account called "Everyone," giving it read access, now click the box that states to apply to all child objects (files and folders) and click Apply/OK.
Now you should be all set.