Game problem or computer problem?

[skadooshhh]

Recently, I've bought the Showtime EP, it's been working fine since I bought it (Thursday), since yesterday, it's been messed up(Saturday). When I play my game the graphics would go crazy, there would be triangular edges around the Sims houses. But what's weird is, is that my sister plays the game as well, she has her own account and I have mine. Her Sims plays perfectly - no glitches, no messed up graphics... Is this a game problem or a computer problem? Has anyone encountered this problem before? If so, could you please help? I really don't wanna spend money on something that could be simply solved. If it makes any difference, the computer make is an HP Pavilion Elite HPE -250f 2010. Is it time that I updated my graphics card?

Please, please, PLEASE help!