did you format the drive in which vista was installed?
remove all the wire connected to hard disk and replace them and try again

hope this works
good luck

it is still running Vista now..it is when i format it using XP disc when it says it can't see any hard drive installaed. when i boot it from hard drive, vista boot successfully...i wonder if it is XP which can't take out Vista!!

how are you tryin to format using windows setup or boot the disc and use dos mode to format

Why do you want to do this? Vista is a fine OS. If you're having problems with it, ask, and we'll help.
Have you found drivers for all your hardware? Some machines can't downgrade due to a lack of XP drivers.
What hardware do you have? XP doesn't recognize SATA drives, so you have to follow the "Press F6" prompt to install the drivers from a floppy, which you may not have.

i insert a CD of XP setup and i boot from DVD/CD drive, then it'll load all OS files. when all files are loaded it will ask whether you want to repair or install a fresh copy of OS. then if i want to format i hit enter to install a fresh copy, before it install it format the drive...but my problem now is when i hit enter it says it can't not see any hard drive.

how do i do this one?

the problem is that it is running on a minimum requirements so it is too slow!..damn slow!

assuming ur winxp disk a bootable disc and contains the file format.com
example ur cd-rom drive is D:\

type DIR D:\FORMAT /S
once found use the CD command to tat specific path and type FORMAT C:
that should do it but as hughv said if you hav sata drives then u will need drivers thus floppies

goodluck

no the problem is that XP does not have native SATA support

In your BIOS set the SATA to AHCI/IDE Emulation Mode or alternatively put your chipset drivers on a floppy and hit F6