After you run the speed test, click on the More Details button, and if comment right at the bottom states - Your PC/Workstation has a____ Kbyte buffer which limits the throughput to __ << lets say a low number
Download and run DrTCP from
www.dslreports.com.
Change the The TCP Receive Window to a large number >> say 128578
Scroll down to Adapter settings >> pick the correct adapter connected to the broadband modem> and change the MTU to 1500 for cable or 1492 for DSL. There is FAQ for DrTcp - please read
These are are really registry changes only and they take effct only after a re-boot.
Report back with results or problems.