Yes thank you Emi and Loja.
I found out that my mother card was damaged. In fact this is a common problem on this kind of PC. I can send it to the builder and try to get it fixed for free, but knowing this is an old computer the other solution would be to buy a new one with Windows Seven. The exams are in 2 weeks, I will focus on them and choose what I do afterwards, so I'll have to wait a bit.