I set up Openfire sync'd with LDAP, using Pidgin as the client, for my company. Very easy to set up and supports everything you asked for. :)

I see this kind of stuff about 3-4 times a month between clients and friends. Malwarebytes Anti-Malware is the only program I've seen that removes it easily, and within 5 minutes, to boot. Happy cleaning! http://malwarebytes.org/