You have set up 3COM Boot Services. When trying to boot a machine via PXE, you select "Network boot" and receive the error "Selected boot device not available".
There is no DHCP server on the network.
The above error can occur when no DHCP server is running on the network. The Proxy DHCP setting in 3COm Boot Services does not replace the need to have a DHCP server; it only allows PXE to be ran on a machine other than the DHCP server. You will need to set up a DHCP server on your network. If you install it on the same machine as the 3COM Boot Services, you will need to uninstall 3COM Boot Services, then reinstall after the DHCP server role has been established.
There may be other causes of this error.