internet sites/ e-mail

Here is the problem: I have 3 computers connected via a router. 2 of them are working just fine. I can access the internet and send and receive e-mail without any problems. On the 3rd computer however, I cannot send e-mail, nor am I able to go to any secure internet sites. I am running windows XP professional. I might should mention, that this problem started when I caught a virus and had to reinstall certain programs again. I also called the ISP and they told me that it might be an encryption problem on this machine. Any idea how I could fix that problem? Any help is appreciated. Thanks