I have a Windows 2000 box that was working fine with DSL and cable but when we moved and got FiOS, it didn't work. It has a RealTek RTL 8139(a) PCI fast controller built in to the motherboard that is connected to the port of the back of the Westell 9100em router. The symptoms were very strange: I could ping the router and load the router web page. However, I could not load any webpages outside of the router. DNS was working (lookups were fine and fast) but it seemed like no packets ever came back to me - it was like the TTL was set too small or something - they just died in the ether. Traceroutes would get about a dozen hops out and then time-out. I figured there was a problem with the ethernet controller, so I switched to a 10/100 Hawking USB-to-ethernet adapter. However, this showed the same problem. Then, I switched to a USB wireless adapter. Again, same problem. At this point, it was a total mystery. There didn't seem to be a problem with a particular adapter and I knew it wasn't a problem with FiOS since I had multiple wired and wireless machines (XP,Mac,Linux) in the house with no problems whatsoever.
I had read on these forums some problems people were having with RealTek 8139 adapters: http://forums.verizon.com/t5/FiOS-Internet/Problems-with-the-Fios-20-5-connection-speed-in-my-area-d... so I did a google search on "windows 2000 realtek fios" and found this: http://www.testmy.net/t-19923 This was exactly the same problem I was having, so I did what was suggested: Uninstall TCP/IP and the Microsoft services/clients from the ethernet adapter, reboot, then reinstall the protocols and services... And it worked! I can now load up web pages with no problems.
Even though I was a 25/15 customer, I was pretty happy to get a 7/7 service (according to Verizon's speed tests) after not being able to get any http connection at all previously. For the heck of it, I tried the speed optimizer tool: http://www2.verizon.net/help/fios_settings/optimizer/ After rebooting the machine, I am now getting 25 down and 23 up on Verizon's test and 26 down and 13 up according to the speakeasy test.
I guess the lesson for me is that FiOS is unlike DSL or cable - you might not be able to just plug your computer in and have it work optimally or work at all. But if you tweak it just right, it works pretty well