Double check this:
Right click the folder(s) that you want to share. Click the share tab. Select your share name from the drop down list. Click permissions. Ensure your xbox user name is present. If its not, add it.
Now click the security tab. Make sure "Administrators", "System", your xp login id, AND your xbox user name are ALL present. If any of those are missing, add them. Except for xbox the rest should be there by default but with all the modifying you have done some of them may have disappeared.
Now kill your firewall if you are running one.
Boot your xbox and give it a try.
If it works, try it with your firewall running. If it breaks now then its the firewall.
If it still does not work, even with your firewall disabled, then check your xbox user passwords (both on the PC and the xbox) to ensure they match (you wont be able to view the password on the PC but you will be able to change it.