Hmm, thats not good. It sounds like the wifi was running on your previous modem, probably with the default network names and passphrases. If you didn't have anyone using your wireless, running up your monthly bill, you were very lucky.
Have you stepped through all of the settings for your Belkin router to enable the firewall, and set up the wireless networks with their own network names (not default), passphrases (again, not default) and security settings. I'm just trying to determine here if its necessary to step you through the various pages and, set all of the items as required and ensure that your network is secure. Please don't take this as an insult as there is a very wide range of user capability and knowledge on this forum, and if anyone needs help with setting up the modem or router, I and others here are certainly willing to help out.
My guess is that stepping through all of the setttings will reveal some small detail, like the wireless data rate setting that is keeping your wireless speeds down.