server security remove malware and secure / server admin

I have a small dedicated server hosting approx 20 sites, server has been operational for three years without problem. Today the host contacted me to say that the server has had hacking attacks originating from it and they are threatening to disconnect the server until the virus/malware has been removed. I need an experienced server admin with security experience to look at the server, find what is wrong, fix it, and close any other security holes whilst there. The information from the attacked server says - "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.96/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.55/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.54/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.53/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.52/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.51/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.50/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.49/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.48/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.47/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.46/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.45/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.44/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.43/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.42/5060 17(0) "2013-01-04 19:57:33" myserverip/5060->xxx.xxx.2.41/5060 17(0)