Internet Explorer supports free certificates

With its last update, Microsoft has added StartCom to the pre-installed root certificates in its operating system. As a result, Microsoft products (such as Internet Explorer) now accept certificates issued by StartCom without prompting the user or requiring any special configurations for the certificates. Third-party programs that use the operating system's certificate memory will also accept the certificates without asking further questions.

StartCom offers free certificates for the signing of e-mails (S/MIME) and for SSL server access, such as HTTPS. Unfortunately, with these "Class 1 certificates", the applicant's email address is generally the only thing tested.

While StartCom has been in the certificate store of Mozilla programs like Firefox and Thunderbird for some time, other issuers who offer free server certificates (such as CACert) are not currently included in the Root CA lists of commonly used programs. Users therefore have to inspect and confirm each server certificate or add the root certificate of such issuers to their certificate store.

The root certificate update is available as an option via Windows Update.

Source:

http://www.h-online.com