Determining Which Ports Are Open

Determining Which Ports Are Open

I'm trying to use the Remote Desktop Connection feature of Windows XP Professional. When I was in Bend, Oregon, it worked great to connect to another Windows box through VPN. Now that I am back home--it doesn't work! Port 3389 is what allows Remote Desktop to work--and I have read that some ISPs block 3389 as a security measure. (And no, I turned off the Windows Firewall--that wasn't it.)

How can I determine which ports my ISP is blocking (while waiting for their customer support to get back to me on Monday)? This website has a port tester that claims to let you enter an IP address, and see which ports are open. Using my external IP address, it claims that port 3389 is closed--but it claims that all ports are closed--and that clearly isn't true, or I wouldn't be able to use a browser.

So, is there a free tool out there that will let me know if my ISP is blocking a particular port?

UPDATE: Thanks for all the helpful responses. I was able to do a remote desktop connection--so it may be because I am using the same address range (192.168.1.*) as the far end.

UPDATE 2: Problem solved; see above.