|What do you mean the video card does not work?|
Are you not getting any display or are you just getting a poor resolution?
It may be that you need drivers for your video card.
Look up your model on the HP Compaq website and check if it has drivers for XP for that Laptop.
Newer laptops that ship with Vista or Windows 7 may not necessarily have supported drivers for earlier OSes such as winXP or win2000 or win98.
If that is the case then you should have stayed with Vista. However, if you are lucky then there may be drivers available for winXP.
It always helps if you tell us the make and model of your system as it makes it easier to give you a better quality informed answer.
The quality of answer you get depends on the quality of your question and information given to support it.
Hope it helps.
☺ When everything else fails, read the instructions.