My hard hdisk has four partitions, and my O.S has been installed in the last par

I'm guessing this wasn't done by design or you wouldn't be writing in. We need more information however.

Are all 4 partitions on the same drive? Do you want the other 3 partitions?

If they are all on the same drive and you don't want the other partitions, simply reinstall windows. During the install you will be given the chance to add/remove partitions. Delete all partitions and create a new one. Install the OS to the new partition. Also, it helps if you only have one hard drive attached when installing windows so that the boot loader and OS install to the same hard drive.