Hi guljo,
I'm not too sure what the problem was but it stopped when I did this:
Boot up in safe mode with all your memory installed,
enter msconfig into start menu search and open, <boot> tab, <advanced options>
then edit the # of processors (1 per core) and maximum memory to what is installed (this is in MB)
Then on restart it was ok for me.
It booted into normal with all 4GB of memory installed.
Hope this helps.