My certificates expired recently on my Exchange server and I had to set about renewing them. Not fun, more on that later. I was going to make a long how to on this but Microsoft has a wiki post on TechNet about this very subject with pictures and explanations.
This is for your internal server certificates, it doesn’t do much with the outside world. You’ll still need a certificate for your internet facing services. This will clear up the certificate errors you’ll get internally with Outlook 2007-2010 though.