in your situtation, it would be best to just share a directory and not the whole drive. I don't know what you've shared so I'm assuming the worst-case scenario.
There are what they call administrative shares and they are always turned on by default unless you've disabled them. If you have admin accounts, ensure that they have good solid passwords, using characters/numbers/symbols and at least 10 characters long. Should also be changed once every 30-45 days. Disable any unused accounts.
For the most part, XP will ask for a password when a share is accessed if the user is not identified and simple file sharing is turned off.