If you need to install a self signed certificate on a windows vista computer then you can follow these instructions -

Visit the website with the untrusted certificate. You will see the warning message as shown below, choose continue anyway.

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On the address bar you will see a message for a certificate error. Click on this and choose view certificate.

You will now see a certificate just like in windows XP except the option to Install Certificate is missing… This is because I.E. no longer runs with administrative privilages - even if you are an administrator.

You can launch IE with administrative privilages by right clicking on the IE icon and choosing run as administrator.

Once you click on “Run as Administrator” you will get a security warning which you must agree to.

You can then navigate back to the secure website and click to view the certificate. This time you will recive the option to install the certificate.

On the Install certificate wizard you should choose to specify where the certificate should be installed and add it to the trustsed root store.

You will get one final warning message and then you will be ready to go