First of all: thanks, its nice, but its not exactly what i asked for.
It changes YOUR MAC address, and not an entry for some other computer...

It seems that 'arp'-command reads the 'real' MAC-address from adapter and discards the one you've given
Cant i bypass that somehow?

I can't really understand why do you want to use a different MAC-address
Im just researching my own home network. chapter is arp spoofing