virtualbox.org

ok well a few things. You have 8GB of RAM on your host. You assigned 4GBs to your XP guest. Two guests opened would be using 8GBs of RAM which is all you have and doesn't leave any for your host or VirtualBox. 3 guests open would be assigned 12 GBs which your host doesn't have. Do you actually have Windows XP Professional x64? It's rare. If it's not, you need to change the VirtualBox template to Windows 32bit.

I would try reducing the RAM for your guests to no more than 2GBs.

thank you guys
sorry for such a newbie mistake

all good
got 3 of them fired up and running with 2GB each

welcome. and no problem. glad it's sorted out.