need help with new computer

my old computer went out so i had to buy another one,
got a comoaq presario 3500+ that was my first mistake
had to take the first one back,the monitor stunk so bad you couldn,t stay in same room
got another one,they traded me wouldn,t give me back my money.
had it 3 days.some of the stuff on the motherboard went out,so yesterday they came with another computer,so far so good,my question is,as long as im on the computer its great but i can get up and go in another room and come back its black again,have to shake the mouse toget it back on,i know you should be able to set it far a longer time,but this one i don,t know how,does any one have a idear