ESXi Installed - Hard Disc not displayed / bootable.

I have installed a custom ISO of ESXi 5.5 with special network card drivers onto a desktop PC.

Having completed a successfull install of ESXi onto a 500gb HDD, the PC reboots, but at bios, the HDD that ESXi is installed on does not show up as a bootable device.

Picking the bios boot menu shows me "Windows Boot Manager", the two other HDDs, and the network cards in the PC... by process of deduction I pick "Windows Boot Manager" but ESXi won't boot.

I don;t really get why ESXi can install on the HDD, but then not see it at boot time?

Can anyone offer advice?

Thanks