|Tricky when you can't even get into Safe Mode.|
Were there options on the startup screen for either:
Last Known Good Configuration or a Low resolution startup?
Otherwise, unless anyone else has any brighter ideas, then you need an appropriate HP installation CD to do a repair installation or factory restore (assuming it has a CD/DVD drive). A Live Linux CD can be used to get your important files onto a flash drive.
Always pop back and let us know the outcome - thanks