upgrade vista pro to windows 7 using oem disc

RichardKit

Honorable
Nov 4, 2013
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10,510
I have Vista pro 32bit retail version, installed on my mothers desktop, which I am building for her using mostly new components. Anyhow she wants windows 7 on the "new' pc so I bought an OEM disc/licence. Whats the easiest way to upgrade the OS without formatting the hard drive.
Thanks
Richard