It could also be a cache issue, try flushing your DNS cache and rebooting your computer.
To flush DNS for a computer running Windows:
1. On your local machine, open a command prompt.
2. Within the prompt, type ipconfig /flushdns.
To flush DNS for a computer running Mac OS:
1. On your local machine, open a terminal window.
2. Within the prompt, type lookupd -flushcache.
To flush DNS for a computer running Mac OS 10.5 Leopard:
1. On your local machine, open a terminal window.
2. Within the prompt, type dscacheutil -flushcache.
Otherwise, it's likely a firewall on the server's end which has blocked your IP address for one reason or another, but you will need one of the site owners to get in touch with their hosting provider to check that.
As far as checking your own firewall or antivirus software (I'm 99% sure this is not going to be the problem though) you could temporarily disable it. Most of the well known ones provide a "Sleep for 5 minutes" option by rightclicking the icon in your systemtray.