I just installed Vista RC1 yesterday. I immediately noticed a drop in
internet speed, So I did a speed test on Speedtest.com and indeed my download
speeds were pretty slow comparatively from what I had before changing from
XP. So I went to SpeedGuide.net and did an analysis of the internet
settings. I told me my RWIN was set to 17xxx. So I used the TCP Optimizer
found on their website and changed had it set the RWIN to 256960. I
restarted the computer and went back and had it reanalysed and it still shows
my RWIN to be 17xxx. I dug into the Registry to check there if it had been
changed and indeed it changed the settings but I still am having about half
the speed I had before and it shows on analysis that the receive window has
not been changed.

This is what I am being told my RWIN is still:

Default TCP Receive Window (RWIN) = 17520
RWIN Scaling (RFC1323) = 0 bits
Unscaled TCP Receive Window = 17520

RWIN is a multiple of MSS
Other RWIN values that might work well with your current MTU/MSS:
513920 (MSS x 44 * scale factor of 8)
256960 (MSS x 44 * scale factor of 4)
128480 (MSS x 44 * scale factor of 2)
64240 (MSS x 44)

I have gone through the registry and in every instance of
GlobalMaxTCPWindowSize it is 256960.

This is what it is telling me to expect because of this:
Your TCP Window limits you to: 700.8 kbps (87.6 KBytes/s) @ 200ms
Your TCP Window limits you to: 280.32 kbps (35.04 KBytes/s) @ 500ms
Consider increasing your RWIN value to optimize TCP/IP for broadband

Any advice would be appreciated as I have a 10gb cable connection and I am
getting download speeds close to telephone modem.