said by caribconsult :

I won't swear to this, but I think under Vista your options are more limited insofar as tweaking. For example, the RWIN size is one of the more critical factors in determining speed, but I don't think Vista allows you to alter this, whereas XP and other systems do.

You might have a look over at »speedguide.net. They have pretty good on-line analysis tools, as does DSLR right here, but RWIN is one of the variables you can't change, I think. I'd like to see some verification or refutation of this from some Vista users