Problems with "Security" Properties Tab

I'm trying to make changes in the "Security" tab of the "Properties" dialog on a Windows XP (SP 2) hard drive.

Administrator is set to full access, which is fine. I need to add Network, so that I can access the drive from other computers on my home network. I can add Network, but when I make changes (such as allowing Read Only access) and then click Apply or OK, the Properties dialog freezes with the Apply or OK button stuck in the "pushed in" position. The only way to close the Properties box is to click on the X in the upper right, which crashes Explorer.

The setting that I'm trying to change here (allowing read only access over the network) is obviously stored somewhere, such as in the Registry or in a Security Profile. So my question is: what do I need to change in the Registry or Profiles to set this value manually (thus bypassing the explorer crash)?

Also, a related question: Why do I even have a "Security" tab on this drive? I have 7 other drives, none of which have the Security Tab, and they all work fine over the network. Is there a way I can simply get rid of the Security Tab, and handle Sharing via the Sharing tab, like I do on all of my other hard drives?

Thanks