I have purchased an hp550 computer with the Vista operating system. i wish to downgrade to the win XP operating system. how is this done ?

There are a few ways to do this. If you purchased Vista Business with XP Pro downgrade HP would have sent you a disk.

If you do not have downgrade rights, you will need to call HP to purchase XP Professional and then install it on your computer with the HP included instructions and drivers.

The last option is to return the notebook and buy the notebook model with XP Pro downgrade rights included.

Greetings.
There is no direct path to downgrade Vista to XP. You can’t simply insert the XP installation CD media into the CD-ROM drive and hope to ‘upgrade’ and replace the Vista with Windows XP usual install routine because Vista won’t allow it to be overwritten. XP will treat Vista as ’superior’ operating system and refuse to install over it. There’re actually few options available instead of completely uninstall Vista as listed below. But if you decide to downgrade from Vista to install XP, there is also a simple guideline that you can observe to ensure that the migration goes smoothly.
Option 1 – Dual Boot:
It’s by far the easier and best method to run XP instead of Vista if you have another partition or hard disk. Simply boot up any XP setup media and install XP into different partition or hard drive than the Vista installation. Once completed, you can choose to boot up either XP or Vista, and has the best of both world.

Option 2 – Virtual Machine:
Install VMware Server or Virtual PC 2007 (both free) or any other virtualization software, and then install Windows XP into the virtual machine
environment. You will get XP which runs inside Vista, with minor compromise on speed, performance and have to go through Vista first on every bootup.

Option 3 – Format the Hard Drive, Wipe Off Vista and Install XP:
For those who totally fed up with Vista and just want XP, or prefer a clean installation of OS has only this option to rely on. To downgrade to XP from Vista, you have to basically delete the whole hard disk that installed with Vista, and start anew with XP. Here’s a simple guide and procedures that you can try to follow to switch to XP. However, each system configuration is different, so you may have to adapt the steps to suit your own need.

1. Download all drivers for your system for Windows XP from the manufacturer’s website(in case if you don't have the CD/DVD installers). Copy the drivers files to a USB flash driver or external hard drive or burn them to CD/DVD.
2. Find a Windows XP installation CD (should be pretty easy to find or else you can search in torrent sites such as Demonoid to download via BT network such as uTorrent and BitComet or search and download via ED2K network client such as eMule. You can also use Volume Licensing (VL) CD, retail (FPP) disc, or system builder hologram CD provided by Microsoft.
3. You will also need a XP product key even if you legally can downgrade from Vista Ultimate or Business edition. The Vista serial number won’t works with XP. (try to search for XP product key on web).
4. Backup to USB flash disk, external portable hard disk or CD/DVD every data files and any other things such as documents, downloads, pictures, movies, programs and etc that you need and want to keep. You can also create a second partition with disk management utilities such as Acronis Director or Partition Magic, and then move the data files to the new partition.
5. Ensure that also you have CD or DVD setup media for the software applications that you want to install and use after wiping off your system from Vista.
6. If your OEM doesn’t provide you with recovery CD/DVD discs and Windows setup media discs (it’s because they’re stored in the hard disk in hidden location), you may want to burn a copy of recovery CD to CD/DVD first before you proceed, just in case the hidden partition is lost and you want to recover your computer to original state. (How-to Guide: IBM/Lenovo ThinkPad recovery CD)
7. If the system has SATA (Serial ATA) hard drive, locate the driver for SATA controller and put it in USB flash drive, diskette or CD first. If you have PATA (normal IDE) hard disk, simply go to BIOS CMOS to disable SATA or Native SATA mode if you don’t want to install XP on SATA hard drive. Else, you can also create an XP install disc with the SATA drivers integrated by using tool such as nLite.
8. In the BIOS, also set the CD or DVD optical drive as the first boot device in the boot sequence.
9. Put in the Windows XP installation CD into CDROM drive and boot up the computer by using the CD. You probably need to press a key when prompted with “Press Any Key to Boot from CD…”.
10. If you’re using SATA hard disk with driver on external media (not integrated into XP setup disk), remember to press F6 when you see the message at the status base: “Press F6 if you need to install a third party SCSI or RAID driver…” during the very initial stage of setup to load the SATA driver so that the SATA hard disk can be recognized.
11. Run through the setup wizard at per normal. However, when prompting for where to install Windows Vista, delete (by pressing D and then confirmation), create and format the Vista disk drive or partition (if you have multiple partition). And then choose to install on that just formatted partition or hard drive.

IMPORTANT: If you choose the wrong hard disk or partition with data to delete, you will lose your essential data.

Note: If the setup does not allow you to delete or format the Vista partition or drive, you will need to find a third party software that can recognize and read NTFS filesystem to clear the drive or partition first.
12. Enter any product key for XP. Vista product key will not work.
13. If you own a legitimate Windows Vista edition that allows for downgrade (See Vista downgrade rights), call Microsoft after performing the clean install of Windows XP Pro to activate the XP installation. Notify Microsoft that you’re downgrading from Vista and require to activate XP Professional or other version allowed newly installed. You won’t get a new Windows XP product key though. You will have to read out the long Installation ID plus your original Vista serial key, and Microsoft will provide an activation ID that will make the XP usable.
14. Install the drivers for your system devices.
15. Reinstall any software programs that you want to use.
16. Move back the data files and documents.

Thanks