TCP Optimizations for high speed FIOS

Hello,
I got a new Fiber line from Verizon with 50 Mgs downloads and 20 Mgs
uploads. When I ran the test I could see only 4 Mgs. Verizon had a registry
key that optimized the MTU and Windows size in TCP/IP properties. This
registry fix works fine on XP and Windows 2000, but not on Windows 2003

I ran accross many TCP optimizers on the web, but non of them actualy worked
on Windows 2003. I am currently dual booting. XP shows 46 Mgs, and Windows
2003 shows 4 Mgs. Verizon supports only XP and Windows 2000 workstation.

Please help - Thanks - Mike