Here's what GRC says about port 135:
(Remote Procedure Call) This impossible-to-close port appears in most Windows systems. Since many insecure Microsoft services use this port, it should never be left "open" to the outside world. This port has been exploited to send "Messenger Spam" pop-ups to Microsoft windows users. Since it is impossible to close, you will need a personal firewall or NAT router to block it from external access. Do it soon!