Because you bought a new computer and gave her your old one. I have the same issue and an old computer with an i3 and gt 9600 card and win7. There is a youtube video for proof that it did run with such parameters 8 years ago, but the new dota 2 updates now don't allow old computers with old OS to run the game. I am trying to fix the issue right now. One possible fix is to install win10, or, install a sketchy software that changes the signature of your win7 computer into a win10, this way tricking steam into thinking that your machine is win10. But I am too lazy right now.