Problems with running Exchange 2010

I am trying out Exchange 2010 VHD and running it on VirtualBox 4. x (in Ubuntu 11.04 64 bits). It was working well in the beginning but the computer asked me to reboot a few times. After rebooting several times, there is now a solid blue background after logging in. I pressed Ctrl-ALT-Del and was able to find the Task Manager as well as run applications through the Run Dialogue box. I am able to run cmd.exe, services.msc, and Exchange shell but not iexplore.exe and explorer.exe. However, I can see the processes appearing in the list in Task Manager. I tried running in Safe Mode and installed the Guest Additions without success.
Please help me identify the problem.
