If you want to use Microsoft Hyper-V or Oracle VirtualBox on a Dell OptiPlex 755 system you will need to make/confirm  four (4) specific BIOS setttings.

In the BIOS Setup [F2] change or confirm the following settings if you are receiving errors that the computer does not support virtualization.

1. Performance -> Virtualization = On
2. Performance -> VT for Direct I/O Access = On
3. Performance -> Trusted Execution = Off
4. Security -> Execute Disable = On