You've already referenced add-ons, so that's a positive. And yeah, flash is buggy...sh*te.
Have you examined the currently running processes? Some convenient places to look up info on processes would include:
Neuber Software: fairly comprehensive list.
LiUtilities.com: Another nice comprehensive list of running processes.
Then go through the running services for the system:
Easy reference for Windows 7 (since I'm guessing both machines are running 7)
Black Viper's Windows 7 Service Pack 1 Service Configurations Covers all the standard stuff with Windows 7. 7 is nice in that it frequently includes descriptions of what something does in the services panel.
You may also want to check in the registry at HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{random CLSID (ie. 48B64B72-C34D-2435-3FDD-4598C557FF7B) and then look for the NameServer key and see where it points.
That's for each individual adapter, those may be fine, there's also a NameServer key under parameters, which overrides the others, and typically should be blank.
Obviously backup first if you're really uncomfortable with cruising around the registry.
Scanned with Avira, it's alright, at least it wasn't AVG. You could try AVAST free edition or Microsoft Security Essentials. Depending on the situation, it might not be something that those things would pick up, a simple registry key change is one thing, a running process is another.