This should only happen if you've done an upgrade. If you are wanting nothing from your last OS (files, pictures, music, etc.) then yes, feel free to delete it. However I could be wrong but I believe .Old folder is nothing more than useless files from your upgrade. I always suggest a clean install when switching OS's.
I always delete the .Old file whenever I see someone doing an upgrade. Music and pictures can always be retained and are best moved to a USB thumbdrive before putting an ugrade in store.
And like the other person said, use your basic clean up system that came with your OS. It should be Disk Cleanup. Delete shadow copies and useless files and it should remove the .Old folder without hassle.