You need to grant access to the ASPNet account to that directory, to this in Win XP, you must first turn off "Simple File Sharing".
to do this:
In Windows Explorer - GoTo - Tools >> Folder Options >> Click the "View" Tab
and scroll to the button. Uncheck "Simple File Sharing"
now you need to set the permission, so you right mouse click the directory you want t o grant permissions on and go to the Security tab, and click the add button, in the text box type in ASPNET and press ok, the ASP.Net user account should now appear and you should be able to grant and deny access as you please.
Hope that helps,