Laptop faulty

hi i recently borrowed a laptop from a friend untill i got a new one and it worked for a wile then i went away for 3 days and now it wont work. it is a fujitsu siemens c-series lifebook. the thing thats wrong with it is sometimes when you turn it on it loads fine untill you log on and then it freezes so i unplug it and try again only this time it loads to the windows xp page and freezes so i turn it off again and try it again and now it turns on the fan starts and the lights come on then they go off, and the screens blank aswell any ideas coz i have to give it back at weekend. and when i turn it on when it does work a message came up saying windows\system32\config\system failed or corrupt. please help