It's a standard address - c:\. Their server doesn't see it though (put c:\ into the address bar in IE and you'll see your drive as well).
That information is not transmitted back to their server. Just a little bit of Fear, Uncertainty and Doubt.
Doesn't work in Opera. I don't know about Netscape.
Edited to say that you can type c:\ into Opera and it will list your drive, but invoking it from a webpage doesn't work. Regardless, they see nothing. So unless you're running some kind of wide-open share, nobody's browsing your hard drive, and they're not using a trojan to get that effect.