I personally had a terrible time using VirtualBox on a Lenovo T420 running Windows 7 x64 about 2 years ago. I was having frequent blue screens, so Lenovo sent

I personally had a terrible time using VirtualBox on a Lenovo T420 running Windows 7 x64 about 2 years ago. I was having frequent blue screens, so Lenovo sent out a tech to swap the motherboard. The issue persisted, and after some time, I noticed that it seemed to always occur when I had multiple VMs running. The problem went away when I migrated to VMware Workstation.