Why can't my firefox load any pages while downloading?

I've been using Mozilla Firefox for over a year and a half now and it's been great. But for last few days I've been experiencing a problem. Whenever I'm downloading files firefox can't load any pages. Neither can internet explorer. But the internet connection says that it's connected, the messenger is working, and the file is downloading. In fact, the download speed is much faster than usual. Before the problem started occurring, the download speeds were around 24KB/S, now they're 45KB/S. This started happening after I installed Spybot Search and Destroy and did the first scan (and removed 67 infected files) and than immunized Firefox and Internet Explorer. On the same day, I also increased the half-open TCP connection limitation (TCP/IP limit) from 10 to 60.