Following is a repost of a long reply I gave to another member's question. Hope you don't mind the cut and paste, but it's got a lot of the info you need.
I successfully removed Vista from a new HP Pavilion a1520n and got it completely working with XP. Some of what I found may be unique to HP's specific setup, but what I learned may give others some clues about what to check to install XP on similar systems. Others are welcome to add their tips and experiences.
HP and Compaq no longer provide restore disks. Instead, their systems will create ONE set of DVD's or CD's which is bonded to the machine that creates them. Other manufacturers may do the same. Therefore, before you start, make sure you have your restore disks. You may need to restore Vista for warranty service.
DO YOUR HOMEWORK. Before I started, I searched Google for the HP model number + install + XP. I found that a lot of people have tried to do it and failed on this particular machine.
Originally posted by: Sbuddie
The Dell did come with two partitions and I'd like to keep it that way, if possible.
1. One important thing I learned is that
Vista does some strange things to the partition table. The HP also had two, one of which was used for restore purposes. It has to go. See below for ways to do that.
2. In the CMOS, look to see if the drive is set for ATA or RAID. If it's set for RAID, change it to ATA.
I used Partition Magic, booting from a floppy, to remove them. As it was booting, the program gave me a message that something was wrong with the first partition and asked if I wanted it to fix the problem. I clicked "Yes," and that was the end of that problem. I could then remove both partitions and set up a single, active standard NTFS partition.
If your machine doesn't have a floppy, you may be able to do the same thing booting from a flash drive with the program. You can use HP's DriveKey program to make a bootable USB Flash drive and copy the command line version of Partition Magic to it. I used to be able to find it on HP's site, but
here's one link for it. The zipped file also includes basic DOS commands you may need.
Here's another link on HP's site that may do the same thing. I haven't tried this one to verify it.
A good XP installation doesn't need the restore partition, but if you want more than one partition, you'll have to set that up after you remove your old ones. Partition Magic will do it very easily.
If you don't have Partition Magic, you can try
Delpart.exe. It's a genuine Microsoft utility from NT 4 that will allow you to nuke an NTFS partition from a bootable DOS floppy. I've used it, before, and it works, but I don't know if it will with Vista's partition setup, and I don't know whether it gives you the option of deleting one of several partitions or deletes all partitions at once.
In any case, the objective is to blow off all partitions created by Vista so XP sees a completely new, unpartitioned, unformatted drive that will allow you to create a new, standard NTFS partition.
Be VERY sure you have all the drivers for the chipset and any other features on the motherboard. Find and download everything you'll need for XP, and burn it to a CD before you remove Vista. Check HP's site to look for any drivers, etc. that will work with XP. You may be able to find earlier models using the exact same motherboard that were sold with XP. If so, those drivers may work for your installation. Note the chipsets used for onboard LAN, sound, etc., and check the chip makers' sites for their chip models.
One way to find them is to check the page for the motherboard to see if they list the model or stock number. Then, go to the page where they sell replacement parts, and do a reverse lookup for the part number. It may list various models that use the same motherboard. You may also find info about the motherboard make and model by searchng for it on Google.
You should also download whatever firewall, spyware blockers and any other safety utilities you want and put them on the CD.
Then, when you install XP, do NOT activate it right away. That way, you'll be able to copy install your anti-virus and other safety programs before you ever hit the web to activate XP.
Norton Ghost or Acronis True Image will save your butt any number of ways. To avoid killing the machine, I installed XP on another drive and Ghosted it to the original drive only after I had it running. That way, I could test the installation before blowing off the original Vista installation.
Even if your current drive is SATA, you can build your installation on an old PATA drive and use Ghost to transfer it to a SATA drive once it's working. If you have two spare drives, you can Ghost your build as you get each step running. That way, if the next step fails, you can put it back to the last good point and try again. Running a good registry cleaner, such as Norton WinDoctor or Registry Mechanic, after installing each program can also help keep your build clean and solid.
Good luck.