Why does minecraft keep crashing? - 2

I downloaded minecraft for my 9 year old cousin, it works fine and all but it keeps crashing over and over. It's the 1.7.9 version. First of all, her sister's computer (the one i downloaded minecraft in) is really messed up. It has a virus to begin with. Secondly, it's really laggy. I told her she has to accept the fact that if her sister's computer isn't functioning correctly, then nothing will! Is it probably because the computer is messed up that it keeps crashing? Please answer, she's really desperate to know what's going on. And she's bothering me like every 4 mintues. Thank You for your time:)

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