When using the popular vCenter Converter tool for performing a Physical-to-Virtual (P2V) or Virtual-to-Virtual (V2V) migration of Microsoft Windows, the MachineGuid (HKLM->SOFTWARE->Microsoft->Cryptography->MachineGuid) can change based on your migration settings.
By default, the Reconfigure destination virtual machine option is selected to help ensure the converted virtual machine will properly start in the destination environment as outlined in the vCenter Converter documentation.
If you wish to preserve the MachineGuid, then you simply need to uncheck this box before starting the migration. For modern versions of Windows, this should not be a problem but if you are converting older releases, you should verify that converter workload will startup properly before utilizing this setting.
Furthermore, if you simply cloned a Windows VM in vSphere, the default behavior is to generate a new BIOS UUID which directly affects the MachineGUID. To prevent this behavior, you can add an additional VM Advanced Setting to "keep" the original BIOS UUID as outlined in VMware KB 1541 and this behavior is simliar to de-selecting the vCenter Converter setting based on my quick test.