Dogmush speak true.
Two identical machines here in the house, one running WinXP Pro SP3, one running Win 7 Pro SP1. Same hardware, different OS.
With our Charter 30MBps service, the latter will show 60MBps on Speakeasy Speed Test, the former 20MBps.
Dunno if it's different WinSock implementations, better NIC drivers, IE9 vs. IE8, or something else, but it's definitely noticeable.