What is software piracy? Software piracy is the illegal copying and/or distribution of a copyrighted software program. Use or even possession of an illegal copy is also software piracy.
Types of Software Piracy Online Auction Piracy, which takes several forms, such as: Software resold in violation of the original terms of sale. Academic sold as commercial, NFR (Not For Resale), volume licenses, or OEM software is never authorized for resale by the third party.
"what if i sell a pc i no longer want with its os still installed and give away the backup cd? "
You're not allowed. Technically speaking you should wipe the OS from the PC as the license you signed up to is tied to you or your buisness.
Before you say I didn't sign anything, when you got the PC or installed the OS it asked you to agree to a license. If you didn't read it in full that's your problem ;)