What operating system is on this computer now? A 500 gig hard drive can be had for around 70 bucks. If the computer already has an OEM version of windows then it wouldnt be neccesary to buy another copy if your just replacing the hard drive. If you dont have the media then you can request it from the computer manufacturer. Most of the time theyll just charge you a small fee for shipping it out to you. Typically around 10 dollars. On customers computers we hardly ever upgrade since its more economical for the customer to replace the pc as opposed to upgrading or replacing parts.