1. You can delete and format the entire C and D partions and THEN install Windows XP, in the usual way. But this way, you ll lose the Vista.If you want the Vista you have to re-install Vista AFTER the install of Windows Xp, preferably on a separate partition.Make sure you have all the required Windows XP drivers though...Otherwise most of the facilities of the laptop will not work.
2. The other complicated way is to basically HIDE the partition containing Vista and then make a NEW primary partition and then install XP on the NEW partition. This is a VERY complicated method and would require a lot of research on your part before you successfully implement it. Otherwise it'll give you a lot of confusion and headache.So I won't recommend this method.
3. Make Recovery DVDs of your laptop.THEN follow the method 1 which I gave above.This is the idea I would recommend.This way, you can always fall back on the recovery DVDs if something goes wrong.
4. Whichever method you decide to follow, an EXCELLENT idea would be to FIRST go to the hp website at hp.com and type in the exact model number of your laptop and read up on the issues faced by the other users, especially when downgrading to XP.
I would like to remind you that HP does NOT support downgrading to XP.In other words, the Support Staff are NOT "COMPELLED" to assist you in this process, though most of the time they DO help you with the drivers etc...So its better you make sure you have EVERYTHING you need BEFORE you start installing the xp, by going through that site...
Good luck...