i have vista and trying to downgrade. i bought the xp retail and when im trying to format my parition it shows no partitions.

It shows no partition.
A harddrive can be a single or multiple partition.

With that in mind. you have to enter the BIOS (setup) by using the appropriate key (F1, F2, DEL, F11, F12 etc,etc,etc,) when you first turn on your computer and find the boot order and change it to boot first to CD -Rom (DVD combo) which ever you use. You must save that and exit.

Then Boot to the CD and follow the prompts.

one of the prompts that you may need to pay attention to is when asked about third party or SCSI drivers to push F6...

If your computer uses a SATA harddrive then you will need to install the third party driver for the SATA disc controller at that time.
To do this you will need to have the driver on a floppy diskette if you have a floppy drive installed or on a CD if you don't use a floppy.

you can download the sata driver from the computer manufacturer's website under SUPPORT/DOWNLOADS for your model.......while you are at it download all the drivers for your hardware under the XP operating system as you will need them after the installation.

Once the third party sata controller (or RAID if your computer uses an array) is installed then set-up should be able to recognize the harddrive or partition.

this is a detailed "How To" and tells how to do the drivers and then install.

How to Install Windows on a Serial ATA SATA Hard Drive - SATA Walkthrough

SATA Hard Drive - Installation, Drivers, Setup and Troubleshooting Guide - No Floppy Disk

how do u kno if your harddrive is ATA? - my harddisk is HDT722525DLA380
and my dvd drive is ATAPI DVD A DH16A1P ATA DEVICE

All drives are now manufactured as ATA.....what you probably mean is how to tell if you have SATA (serial ata) or pata (parallel ATA ) which is talking about the interface connection or how the drive is connected to the motherboard.
you can look at how it is connected
1) flat cable(or heavy round) with 40 pin connector is IEDE or PATA
2) either a very thin single connector or a dual connector (1 for power connection) is SATA
or
you can check the BIOS (setup) and it should read :
primary master or slave connection shows it is an EIDE od PATA
if shows "first connected device" or similar it is SATA
or
you can do a search for the model number of youd computer and it will generally give you the details as to what controller is being used.
or
you can do a search for the model of the harddrive and it will generally tell you what configuration is being used.
Support | Deskstar T7K250 datasheet
shows that it is indeed a SATA drive.