Network Bridge adapter changes MAC address

[ssce]

Hi,

I'm using VirtualBox (running on WinXP Host) and I need to set the MAC manually.
However, if I go to
Settings -> Network -> Adapter 1 -> Click on the Additional Options Icon of "Attached to: Bridged Adapter" and change Guest MAC Address.

Those options are disable. Thus, I'm not able to change it.
Any suggestion on how can I set the MAC manually?

Regards,
ssce

[Perryg]

That would be the network section advance sub-section
Be careful. Like MAC addresses will stop the network.