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.
You've got some sort of hardware problem if it really is the memory that's to blame. It would probably be a good idea to run MemTest86 or some other good memory diagnostic to see if it reports any problems.
Is the new memory the same speed and voltage? Have you checked your motherboard manual to ensure you're putting the pairs in the correct slots?