How to install Windows7 64 Bit on Travelmate B115? (Error 'Security Boot Fail')

Hi!

today I bought a Acer Travelmate B115 (M41 RQ).

I has a rudimentary Linux installed, which I want to delete and then install a Windows7 Prof SP1 64bit from DVD.

After changing the order of the boot devices and putting the USB-DVD drive first the Travelmate tries to boot from the DVD but then tells me 'Security Boot Error'. I tried it more than 10 times, same error each time. The external USB-DVD drive is working fine on another computer.