I'm using 4 mbps ADSL line with a US Robotics 9108 router at my home with 3 devices connected to it. 2 laptops, one with wireless and one with wired connection, and also a PS3 with wireless connection.
Problem began when I noticed I was getting slow speeds with the wired laptop, and I ran a speedtest at different times during the day which were between 600 kbps and 1 mbps. I ran 2 different anti spywares and also 2 different anti virus scanners, cleaned some malwares and trojans but speed stayed the same. Then I noticed PS3 was suffering from this slow down also, with games being laggy to a point that you can not play online. ( I did a speed test from the PS3 browser, which showed the same result as the laptop.)
I probably need to mention that i set the laptop's and PS3's ip addresses manually and assigned static ip's to them.
I called my ISP and they said that my line was not the problem as they saw constant 4mbps on my line, which I also saw on my router's config page.
Now the weird thing is the wireless laptop is not affected by this slowdown a bit. I even unplugged the ethernet cable from the wired laptop and plugged it to the wireless one, ran the speed test on it with a result of 3.2 mbps...
I am at this point at a loss as to what is causing this slow down on one laptop and the PS3 and how it is not affecting the other laptop...
try running the slow laptop in safe mode with networking and run another speed test, and when testing try make sure only a single device is on the network at a time, as software like windows norton and stuff have been knowen to download stuff without the owners knowledge. And every time you connect it maybe hogging a bit of the line.
If you get normal speeds in safe mode it'll be a software issue, and I'm not gona bother explaining how to fix it, cause its alot more writing I'm sure you'll find it on google somewhere though.
You can also try a traceroute from both laptops to see if there taking the same paths.
also have you tried different speed tests sites? they may not always be accurate (or close)