i checked the HP support website and according to HP's policies on bios passwords, they offer a paid service for the password removal A Password is Required Causing the Computer to not Startup or Boot in Windows XP HP Pavilion dv4000 CTO Notebook PC - HP Customer Care (United States - English)
**see the section of "Power On"
i read in some discussions that the service costs ~$35.
if the laptop is no longer under HP warranty, and if you want to take a chance at removing the cmos battery to reset the bios password:
1. Take note that i could not find any manual/guide from HP on how to locate and remove the CMOS battery.
2. However, this video for removing the wireless lan card of of the laptop seems to reveal a container for the cmos battery.
3. Follow this video tutorial for removing the wireless lan card. Steps for Removing and Replacing the Wireless LAN Mini PCI Card HP Pavilion dv4000 CTO Notebook PC - HP Customer Care (United States - English)
4. After removing the wireless lan card, see if you can detach what seems to be the cmos battery container
5. remove the cmos battery, let the computer idle without the cmos battery for 30 seconds.. and still without the cmos battery, power on the laptop. it might give a cmos checksum error. hopefully this error is a sideeffect of the cmos battery being removed and the bios password being reset.
also there are some third party services that offer bootable cds that promise to be able to reveal the cmos password or simply remove the existing password. PC CMOS Cleaner, Access your computer now.
**if you are willing to pay for the removal of the bios password, i suggest you check first with an authorized HP service center, and then compare the price with third party services.