The best solution I can give when that dreaded blue screen appears is to reformat your hard disk and make a clean installation of OS. I suggest you install Windows 7, it is far better than Windows Vista in almost everything. I think your recovery disc includes the Windows 7 OEM that comes in the HP package.

In my knowledge, Windows 7 does not support downgrading. So I assume that your attempt to downgrade your OS to Windows Vista causes your system's trouble.