Strange pattern

In my previous post I had said tat my computer had crashed, but its not so, it suddenly sprang to life on friday. I dono how it started working.I was very happy that it had started working and all.But wen I had just began working on it, a black screen came in and then strange noises from the system started coming, finally wen I switched it off manually, I expected to restart properly.....surprise, It gave me OS not found errors(GR8)....But after an hour or so wen I switched it on,it started working as though it had no problems wat so ever..Even now it shows these errors very often than never.
If someone had similar problems tell me wats wrong here, or if u hav a valid reason for this kind of a problem pppppppplllllllllsssssss tell.(I am desperately lookin for a way!)