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Windows 8.1 has been running fine in VirtualBox running on Window 7 Profession, but now will not load. My log is attached.

I'm not positive but you are getting host memory low errors. Your guest is set to 4GB RAM but your host according to your log has approx 4.7GB available. Normally you don't want a guest to be set for more than 1/2 the host's available memory.

Did you install any new apps in your host or guest?

You could try reducing the guest's memory to something more reasonable.

As a rule of thumb, don't give a VM more than 50% of your host's available memory. In this case your host has 8GB installed, 4.7GB available, so giving 4.128GB to the VM is probably a bad idea.

You might also investigate why your 8GB Win7 host only has 4.7GB available. Are you running another VM?