Ok, bascially delete the one that is NOT your operating system, to do this, right click 'my computer' click manage, click disk management, now right click your other OS and format it.
Save things that you may need before formatting.
Alternativ way would be to put your OS disk in and restart your pc, now tap del or f2 and a blue screen will appear, now look for a menu where your boot order is, normally its advanced bios features or boot menu, now choose your disk as number 1 and harddrive 2, now press f10 / save and press Y for yes to save and exit, now it should spin your os disk and a black screen appears saying press any key to boot from cd, press a key and it will load things up, now go threw the instructions and follow ir threw to you come to your partion part, now you can press D on the highlighted one you want of then press L and it should del, goodluck man.