During upgrade my HP DL360 G5 ESXi 5.0 u1 hosts to ESXi 5.1 I received a purple screen with warning message:

 

The system has found a problem on your machine and cannot continue.

VMware ESXi requires the Execute Disable/No Execute CPU feature to be enabled

 

Solution:

Restart the host, press F9 to boot in BIOS mode

Advanced Options > Processor Options > No-Execute Memory Protection, then configure: Enabled

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