> I have no problems with my internet
> connection and the site I want to reach is definitely up.

When you're looking at problems like this, make sure you're using fairly
simple tools.  Don't use browsers, for one thing; something like 

telnet the.destination.system.com 80

to talk straight to the right port on the destination system is a better
choice than a browser since you know telnet won't play games with things
like proxies and caches.

> Regarding Eric Hodel's question, I'm not sure if I am behind a proxy 
> (or firewall for that matter). How can I find out?

This is the bit that makes me suspicious; if you don't know how to find out
whether or not you're behind a proxy of some sort, what did you do to figure
out that the site you want to reach is "definitely up?"  

If you're just testing that you can connect to the remote site by hitting
refresh on a browser, there are possible things between you and the target
that could give you misleading symptoms.

 - James Moore