HTTP 403.16 error

I have downloaded and installed an SSL certificate from www.freeSSL.com for
testing purpouses. I have a WXP SP2 machine with a IIS 5.1.

I configured a virtual directory to request digital certificates when
accesed. I have installed the freeSSL CA root public certificate at the
'Trusted Root Certificate Authorities' of the 'Local computer' certificates
snap-in of the server machine.

To identify the client I am using a digital certificate issued by the FNMT,
so I have made sure that the public key of this CA is also at the 'Trusted
Root Certificate Authorities' of the 'Local computer'.

When I try to access the virtual directory I get:

'HTTP 403.16 - Forbidden: Client certificate untrusted or invalid'

what is it I am missing? Thank you.