Difference in Speed test between two PCs

[#1]

Something odd, and I wonder if anyone can shed light on it.  I've got a BT fibre connection.  The Netgear router is connected directly to a 200AV Homeplug.  The next Homeplug feeds a small switch that has 3 PCs connected to it.  Two of these PCs are Windows XP, and one is Mandriva Linux.  Now, if I run broadband speed tests on each of the PCs (remember they're all connecting to the router via a 200AV Homeplug), the two Windows PCs report about 29Mbps.  However the Linux PC reports 32Mbps.
If I plug a laptop directly into the router (bypassing the Homeplugs) I get a speedtest result of 32Mbps. 
All broadband speed testers give the same result.  2-3Mbps slower on the Windows PCs.  The only other difference I notice is that the Linux PC reports a Ping of 15 to the Speednet servers whereas the Windows PCs report 22. 
Anyone know why this might be?  I'm running a 3rd party firewall on the Windows PCs, but I get the same result on the Windows firewall (and I'm damn well not running it without!).
-Curious Dave

[#2]

What are you using to run the speed tests? I'm just wondering if you're using a java based tester? I'm not sure why but perhaps a difference in the way Windows and Linux use Java might be a starting point for you
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[#3]

What do you have the RWIN size set to?
If you don't understand the question go to http://www.dslreports.com/tweaks and see what it recommends.