You're having memory management problems which could definitely be caused by the two different memory sticks. If this is the case, then lots of bios tweaking would be necessary to get both working together, if at all.
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AVG is quite notorious for memory management bsods too. I recommend to uninstall it in safe mode using the tool found here:
AV Uninstallers - Windows 7 Forums
Reboot to normal mode and install MSE.
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Delete this driver from C:\Windows\System32\drivers:
Code:
pcouffin pcouffin.sys Tue Dec 05 09:39:30 2006
It will break whatever software you have that installed it. That's alright though because you don't want this driver on the system. When you figure out which software doesn't work any longer...