If u want to install windows and it says something with the gpt,just format it from the windows,;
Insert w7 cd,let it load,choose your language,click install now, etc, don't choose upgrade! Choose advanced( i guess,not remember exactly,it I'll let u choose two options,upgrade and one more,choose the non upgrade one.)then somewhere,it supposed to ask u where u want to install..
Choose your drive and press advanced( guess...) then u should have delete or format partition,( I'm do the both..) so do that it's exactly a minute,then u should proceed..to install
If u can't change the sata mode on bios and your sata cables,especially,the hdd!
Good luck and say hows it goes