Sometimes certificate athorities let you just renew. Godaddy does this. I don't know how exactly. So then you get a new certificate with the same private key. You have to use the MMC Cert Snapin to import it. But it won't have the original private key associated with it.
certutil -repairstore my "SerialNumber".
Now it has a private key if you actually have the original private key.
http://support.microsoft.com/kb/889651