IE and Vista - Certificate Issue Problem

[babypink]

Hi, not sure if this is the right place to post but here goes and new to anything computers!

I am trying to access for the first time my company server, via webmail (was given the URL Link) all fine and good. However, when I click on the URL I get the following

Revocation information for the security certificate for this site is not available, so I click yes to continue

I then get a security alert which explains things so click on yes to continue then login

Then I get

There is a problem with this website's security certificate.

The security certificate presented by this website was not issued by a trusted certificate authority.

Security certificate problems may indicate an attempt to fool you or intercept any data you send to the server.
We recommend that you close this webpage and do not continue to this website.
Click here to close this webpage.
Continue to this website (not recommended).

I then continue and login again, get into my webmail but can only read, I cannot create, forward, reply nothing?

It asks me to download certificate which I did, and I can see the certificate but it dont make any difference at all, please help

Thanks