> We too have had cases where NOD32's IMON module caused problems by
> altering packets. We have also had cases where the software provided
> by some mobile phone providers, e.g. Vodafone in Australia, also
> alters packets causing the same sorts of misbehaviour.

My experience with Wireshark is that a lot of the time it says the checksum is incorrect, even when there are no problems (although those checksum errors could also be a result from running behind a NAT router). I wouldn't focus on that as the cause of problems.

Wireshark catches the traffic sent from the computer to the networkcard and not the traffic actually sent by the networkcard. Most networkcards have builtin header checksumming these days so Windows offloads the checksum calculation to the networkcard (leaving the value zeroed out (I think) when it sends the packet to the networkcard). So outgoing packets will usually have the wrong checksum.

I would focus on (software) firewalls (eg Windows Firewall) or antivirus software that behaves like a firewall at times.
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