Seems a little dire to be contemplating a full reinstall just because of a failed install of Power Director 9
You do not need to buy another copy of W7 Operating System. If you have not made your backup DVD that you should have been prompted to when you first got the pc you can download and burn one from the link on http://www.heidoc.net/joomla/technology-science/microsoft/14-windows-7-direct-download-links
It is the Product key that is important and there should be a sticker on your pc tower with the W7 product key. This may be different to the actual key used as big manufacturers use a special bulk key pre-activated for their installs.
However you shouldn't need to go to that extreme, if you really want to start from scratch there should be an option on your pc to 'restore to factory fresh' settings that will take you back to how it was when you first got pc. This is usually stored on a hidden partition on your harddrive and is accessed by various key combinations depending on the make of your pc.
What make and model of pc do you have?
As for fixing the error you get can you give more details of any messages that come up and is everything else working ok?