Mixing brands of RAM is usually not a good idea. The many different timings of the two brands are going to be different and the system will use, what it considers, to be the slowest for all of them. This may not lead to a stable system.
Use the 64 bit version of Windows 7. It is fine now.
Many motherboards cannot support all 4 slots filled with RAM without some issues or without specifying exactly what is supposed to be in al 4 slots. Check the documentation, for the mother board, and see what it considers appropriate sizes and speeds if using all 4 slots.
It still sound like you are having corruption problems to me.