Hi Simplicio.
Thanks for your response. And yes I am using Anonymous access in my virtual folder. I think is trying to use COMPUTER_NAMER\ASPNET account because I was having this error message before granting access to ASPNET account:

Login faild for user 'GUILLERMO\ASPNET'.

If I wanted to use a SQL acount instead, what do I need to set in IIS besides creating creating an account granting permissions?

Thanks is advance.