Terrible slow win7 64bit after upgrade from 2gb to 4gb ram

Hello, My windows 7 64 bit version works usually fast with 2gb RAM, I dont have any problem with speed. But when I insert another module of 2gb to make it work with 4gb ram, windows 7 64 bit becames terrible slow, I wait around 20-30 mins to open start menu. ProcMgr shows that CPU is at 100% of usage, but there is no process that uses so much. There is no viruses, because it is a clean win7 installation to recently formatted disk. If I plug off one memory to leave only 2gb it works pretty good. I have tested both memory modules separately, and both work well.