What has changed between the last time PE9 worked and now?
If you have updated Windows (automatic or manual updates) that will very often create a situation where you need to update your video driver
You may also need to update Quicktime
As John T. points out, an obsolete video driver can cause problems with PrE, including failure to launch. See this article for more info: http://forums.adobe.com/thread/948651?tstart=0
I would also see this article for tracking down crashes and hangs: http://forums.adobe.com/message/2031203#2031203