My computer will restart over and over

Hello Guys ,
I have an unbranded computer and when i was brousing throught the internet it just shutdown and i tried to restart it , but it will restart over and over when its gets to the Windows Xp loading screeen and so i pressed the f8 button and disabled the automatic restart nad the it says that i have a porblem which is UNMOUNTABLE_BOOT_VOLUME
anyone can help to fix it because i have important documents on my harddrive

manythanx