Corrupted OS that needs to be reinstall again

Hi Techyv!
My question is, My computer is pretty old now, about 4 years old, and every time the os gets corrupted or when it is infected with viruses, I just reinstall the os again. Some say I should reformat completely before installing another os, but I have lots of important documents, and pictures in my hard disk and transferring them to the 2nd partition is pointless because of space issues. Every time windows installation is done, I can see a selection of different os that have been collected in my pc. There are 4 of them now. How can I remove the other ones? Is it possible?
Thanks,
Faye Ty
