Try this BogMacUk i had the same problem since i got a new router on saturday (wrt54gl), tried the below, and i am now getting full 10 meg :-
Download DrTCP -
http://www.dslreports.com/drtcp
Set TCP Receive Window (RWIN) to 64240
Select your network adapter (wireless adapter in my case)
Set MTU to 1500 and leave other settings.
Hit Save, Exit and Reboot your computer.
good luck!