Issue with config on one pc on wireless home network

Hi, For the last couple of weeks, i've had rubbish speed on my pc when connected to wireless. Used to get around 6Mbps bandwidth but this has decreased to around 2Mbs. I have 2 other machines (i have not tampered with :/) on the same network which perform fine (around 6Mbps). I tried several 'Speedguide' tweaks (including Nagle algorythm tweak) but get varying speeds and 'rubberbanding'...