loading error on operating system?

something went wrong with my xp sp2 and when i re-started the message "error loading on operating system". This happend before when I upgraded to vista and when problems started to show I reformated, partitioned, and installed my previous xp cd2 and started getting the "error loading on OS". I tried to go back to vista and installe it without a hitch but then I hate it. I reformated again, partitioned and left a 1 gig unpartioned space and installed xp and it worked. but then after using it for a day I re-started it and there it goes again.."error loading on OS".. please help