Are both your pc's hooked up to the router?
Can either of them ping the website you say won't load?
Have you asked someone else outside your lan to test the website to see if they can load it?
About your anti virus software, which one are you using? I use norton and I'm behind a di 704p fw and don't have any problems updating my definitions.
When you try to access the configuration page for your router (
http://192.168.0.1 on my di-704p ), are you typing "admin" in all lowercase? that's the default password afaik.
BTW, why do you want to disable the fw? Just to update your AV software? If so, just disconnect the router, turn off the modem, turn off your pc, wait a few minutes, then hook up one pc directly to the modem, turn on the modem, wait for it to finish booting and then turn on your pc. Now, update your AV software, and repeat the process for your 2nd machine. Once they're both done, turn both pc's off, turn off the modem, wait a few minutes, then turn on the modem, wait for it to boot, then connect the modem to the router, turn the router power on, then turn your pc's on (assuming the router is a DHCP server leasing out dynamic IP's to your 2 pc's) and you should be back to normal.
Hope this helps, if not, be a bit more specific on what your network layout is like atm, and a bit more detailed info on what you're trying to do, and what error msgs you see/don't see, and other possible relevant information to us.