Application using Webservices being blocked

I have an application that uses web services to talk to the DB. Not unusual right? A handful of clients can not get any access to the web service. I believe it has to do with the anti-virus protection software that they have installed, but I am not really well versed in these applications. I can log in just fine from several machines with their login and password, so my hunch is that their machine is blocking me. So my questions are the following: 1. What can I do to log out the error that is happening when they are not able to get a handle on the web service? 2. What should I be looking at to get their machines to log in? (i.e. anti-virus, firewall, etc...) 3. Is there something that I might have set up wrong on the web service or in the application that might make the security software from thinking it is a trojan horse or a virus? 4. Am I way off base on my line of thinking? 5. Can anyone tell me anything that can help me solve this problem? Maybe it is someplace I should call, etc.... Desperate and thanks for your help.