If you really want to trust your computer you really do need a good clean copy of the O/S, , yes you can get any amount of the on-line, but that is also the way "Bots" are developed, - through corrupted o/s`s, unless you are very good at studying and analysing CRC Checksums, you will never know if you have a true copy or a corrupt one.
I personally choose to only install clean and safe.