I'm pretty sure simple file sharing can't be turned off easily, or at all, in XP Home. I have XP Pro, and in that you just open My Computer (or explorer), go to Tools, then Folder Options, then click the view tab, then uncheck "Use Simple File Sharing (Recommended)." But I just checked my boss's laptop with XP Home on it, and that option isn't there.
If you check
this it says: "Windows XP Home Edition-based computers always have Simple File Sharing enabled." under Enabling and Disabling Simple File Sharing.
I have heard that if you boot into safe mode, you can get around it. You can also use the cacls command line tool if you are comfortable with that.