We've had some firewalling issues that even knowledgeable users are having problems with. If the obvious isn't helping, read on:
(1) The slightly-less-than-obvious: Don't try to forward ports 10000-50000 to your machine. That's just BitTornado's random port choosing default. Pick a block of 10 ports somewhere in that range and change BitTornado's settings to those ports. Then set a static IP and forward those ports to your computer from your router. portforward.com may help.
(2) Double-NAT. You may have another router on your network, built into your cable/DSL modem. To find out if this is true, go to your router's configuration menu and check what it says is your external IP. If the IP starts with "10." or "192.168." you either have a router built into your modem or your ISP has you behind NAT (network address translation). You may also want to compare the external IP reported by your router with the result from whatismyip.com. If they differ, you're behind a NAT or proxy.
It may be difficult to get around double-NAT. Often the built-in router in a cable/DSL modem is too basic or has problems. Some ISPs require logging in via PPPoE or other systems, and disabling the modem's onboard router may prevent the modem from logging in for you. In cases like this you may be able to configure your separate router to log in instead.
(3) Doubled security software. If you have both ZoneAlarm and McAfee software on your computer, for instance, it's very likely they will interfere. If you have security software installed, make sure Windows Firewall is disabled.
(4) Proxies. Since BitTorrent (mostly) connects to trackers via HTTP, a HTTP proxy setting on your system will result in the wrong IP being detected by the tracker. Some malware will set up proxies for your computer, tapping your communications or inserting ads. For Windows, check under Internet Properties, Connections tab. Most people will have no need for Dial-up or Virtual Private Network settings, and anything other than an empty box there should be considered suspicious. Also, click the LAN settings button. You may wish to uncheck "Automatically detect settings", and anything under "automatic configuration script" or "proxy server" is also suspicious.
(5) Driver issues with Windows XP. I've come across this a few times, when for some reason XP doesn't set up its drivers properly. Try running the Network Setup Wizard, and make sure you tell it you have a direct connection to the internet.