Need help with unusual Internet speed problem

Hello there,

I have a Wireless router and 3 computers. Let me name them as A, B and C...
A and B are desktop computers. C is a laptop.
Computer A directly connected to the router via network cable. B is in my room and it connected to the router via a Wi-Fi dongle.
Computer C (laptop) is also using Wi-Fi.
Problem is like this. Computer A and C is ok. Computer B's download speed is very slow (the one connected via Wi-Fi dongle). I checked all PCs with speedtest.net.
For computer A and C, download speed is 3.7Mbps. For B only, 0.9Mbps.
I thought Wi-Fi dongle has a problem. But, when I connect Wi-Fi dongle to Computer A, it takes the full speed. Plus, I bought a 10 meter long network cable and connected it between router and Computer B. But, still same problem.
Problem can't be with router, because laptop is working fine via Wi-Fi. The Wi-Fi Dongle also fine. Because when I connect it to a different PC, it works fine. Problem is with Computer B. What should I do? I did everything I can. But, still it has very slow internet speed..
I'm going to format C: drive and install windows again. Is there any solution except formatting? Please Help..

Thanks
Duminda C.