Last week one of the root certificate authorities, Comodo, issued some fraudulent certificates for some very high-traffic and popular domains (including google.com, live.com, and yahoo.com.) The risk is that someone will use these certificates for nefarious purposes.
Microsoft deemed the risk bad enough that they released an out-of-band patch that will revoke the bad certificates. You can download this patch here. I highly recommend that if you are on a Windows machine you apply this patch.
More information about this problem can be read here. A good article about this story is available on cNet.