Outlook Express Error on Security Certificate Verification

Hi,
I face difficulty while checking emails through Outlook. It opens indicating an error about the validity of certificate. It feels miserable to witness those error messages. It becomes a hunting hurdle when I am in urgency of reading or replying an email. My P at home shows these errors to me.
Outlook Express
The server you are connected to is using a security certificate that could not be verified.
A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provider.
Do you want to continue using this server?
At my office, these problems never occur but when I access Outlook at home it shows these errors.
For the answerer’s help I better mention that I am using Windows XP with service Pack 3. Kindly help me resolve this validation issue. Are these certificates necessary to have? What if I don’t have them? What other way out is there?
