i am trying to reinstall Windows XP on my compaq presario laptop, but I cannot, it gives me a message: Hard disk not detected or power on the hard disk.

Why are you reinstalling?
Can you boot from a CD?
Is the HD detected in the BIOS?

i am trying to reinstall Windows XP on my compaq presario laptop, but I cannot, it gives me a message: Hard disk not detected or power on the hard disk.

Dear
do one thing just go to bios and change ACHE/SATA 2 IDE
then try to install

First check your BIOS settings and see if Hard Drive is detected by
your computer or not? If your Hard Drive doesnot show in BIOS,then
turn your computer off, and open the casing,check and see if the data cable is properly connected to the Hard Drive and other end of the data cable is properly connected to the IDE slot in the MB.
Make sure that your Hard Drive is connected to Primary Master ,the IDE Slot 0. Also check the power cord plugged into your Hard Drive.

First check your BIOS settings and see if Hard Drive is detected by
your computer or not? If your Hard Drive doesnot show in BIOS,then
turn your computer off, and open the casing,check and see if the data cable is properly connected to the Hard Drive and other end of the data cable is properly connected to the IDE slot in the MB.
Make sure that your Hard Drive is connected to Primary Master ,the IDE Slot 0. Also check the power cord plugged into your Hard Drive.

its a laptop

You need SATA drivers to install Windows properly or you must disable SATA from Bios and switch to IDE.

Why are you reinstalling?
Can you boot from a CD?
Is the HD detected in the BIOS?

Iam trying to upgrade my OS and
I can boot from CD and BIOS doesnt detect my hard disk.

sounds like you have a sata hard drive and winxp need to install drivers for it if you have them on a floppy then you need to hit f6 in the first part of the install to install them or , try what was suggested in post #3