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One (and always the same) PC blocks Internet access on the entire network. It usually happens Sunday morning after not using it on Saturday, but it could happen any time.

What happens is that this computer uses up all network bandwidth and doesn't allow anyone else to access the Internet. The funny part is that this PC is still in sleep mode and no one uses it.

The way to cure this is to reboot the offending PC; all PCs on the network (including this one) then work normally.

We checked for infections on this PC but haven't found any yet.

Is there anything else I could do to troubleshoot this problem?

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You could run a packet sniffer on the network (ex: wireshark) and record what traffic is going across the network. You can easily enter in the IP address of the offending machine to only view traffic going to and coming from that particular host on the network. IF your PC is doing something like this particularly when the machine has been idle for a long period of time it does sound like something malicious could be going on. I recommend a complete scan of your computer with MSRT (C:\Windows\System32\mrt.exe) as well as using Microsoft security essentials for Anti-Virus if you do not already have products you prefer.

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