comp rebooting while offline

hi. my comp is has windows xp. and my prob is that it keeps turning off during use.not internet capable at the moment, so got two games from daughter inlaw and the comp is turning off just after you start to play. sometimes when using media player listening to music it will just reboot itself.so i play alot of spider solitaire...but its now rebooting during that!! please can you help me?? was seriosly thinking of taking it back to the factory settings ...but dont know how. rang the store where i purchased it but they charging big money to reformat it for me ...i would like to try and do this myself, cos cant afford the store to do it.