Computer causing internet to be slow on entire network.

wicky woo

Distinguished
Jan 30, 2010
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I have 2 computers on a network, and 3 devices that use wifi on the same network. Basically when ever computer A is connected, every single computer or device on the network runs extremely slow, if at all. I get anywhere between 1200-2000 ping on pingtest with 2-60% packet loss. Even the speed on the "infected" computer A downloads extremely slow. I'm at a loss for this problem, everything runs slow and its because of the one computer connected to it. I have virus software on this computer, but its not the best so i'm not ruling out malware or viruses right off the bat.
Any suggestions or ideas?

Devices: Blackberry, Iphone, macbook (all wifi)
Computer A
Runs windows xp 32 bit,
amd processor
onboard video

Computer B
Runs windows 7 64 bit
i5-2500k
gtx 560 ti

The internet modem runs to a router (Dlink dir615) then to a switch (or hub, if you will) and then to the 2 computers. The entire problem is solved by unplugging computer A from the system. But other than that I have no alternative.