Hi TrondS,
I remind you that Windows, during his initial installation, do not like that there is more than one stick of RAM installed in the motherboard.
After the end of the entire installation process of Windows, made a performance test (Experience Index score).
It is in the System section of Control Panel.
After this test, shut down the computer and install your other sticks of ram.
What is your model of ram.
What is your version of windows and type? (Like Win7 64bit, for exemple)
Gloup_Gloup