Sudden restart of system?

i am using windows xp as os. the configuration of my system is amd athlon 64 x2 processor, 1gb ram and 160hard disk. i was using linux but it got crashed so i installed. from then onward whenever i do a certain operations like coping data from pen drive to system or reading any pdfs the system suddenly hangs and restarts after few seconds. my system is virus free. i changed the slot of ram but the problem continues. please help.