Re: Access to theme.skin denied
You need to make the Data folder beneath the root of the web writable by the aspnet worker process.
Right click the Data folder in Windows Explorer and go to Properties > security tab. (If you don't see a security tab, then google about xp, simple file security or simple file sharing). If ASP.NET Machine account is listed, give it full control. If its not listed, then click Add..>Advanced..> Find Now and add it then give it full permission.
Setting the Trust level to low makes security so tight mojoportal will not work. On your xp machine you should leave the trust level entry of web.config commented out so you run in Full trust.
Hope it helps,