Hard to tell cuz your computer is the only one with the issue. I recently banned a broad IP range, that may be it but since you can get on with your psp I doubt it. Go to this site:
http://whatsmyip.org/ and then type your IP address here in this thread, and we can go from there.
Also about the FireFox, did you download it and try to access dmclanforum.com and it didn't work, or you couldn't download the firefox cuz that site didnt work either? Is dmclanforum.com the only site that doesnt work for you? if so, what happens after you see "internet explorer has encountered an error" does it shut down, or just doesn't open the page?