Internet speed ; fault of Win 7 ?

Hello All,
I've been suffering slow speeds, 3.5 Mbps when 17 - 19 Mbps is average.
BT are doing their bit and out of interest I've done some work to see if it could be this end.
I booted into Ubuntu from a memory stick and ran a speed test and the result was 18.5 Mbps. I booted back into Win 7 and got the "usual" 3.5 Mbps.
This result suggests that it is something to do with the OS but how could that be ?
I also booted into safe mode with networking and got a 3.5.
I'm stumped, never heard of this before and any help would be appreciated.
Nelly.
Mods : apologies if this is the wrong place. Please move if needed.

What browser are you using ? Try the test with google chrome.

I've used Firefox so far, with 7 and Ubuntu. I've been asked by BT to perform tests tomorrow so I'll try Chrome then.
Thanks for the quick reply.

Hi Addram, Tried chrome and the results were 19 Mbps which is about right, Tried Firefox on the same site and the result was 2.06 Mbps.,
Odd thing is if I D/L a file from thinkbroadband :: Download Test Files then it takes about 9 mins using either browser. A glitch in Firefox I think.
Thanks for your help.