how do i install xp on an ex- vista machine??

fijitsu siemens amilo laptop
dual core 1.5 ghz
2 gb ram
160gb hhd

Boot the XP CD and follow the instructions for a Clean Install.

If you get any problems you may need to "Wipe" the hard drive first thus eliminating Vista prior to the install of XP.

KillDisk will do that for you: Active@ Kill Disk Hard Drive Eraser. Low Level Format.

You will most likely invalidate any warranty if you do this.