for example Laptop, win7 yung original na O.S. then gusto mo iformat sa win xp Downgrade tawag don boss.
Mostly pc and laptop pagpasok sa bios press "f2 or Del" key for example gamit mo cd or dvd set mo sa 1st boot cd-dvd drive, if gamit mo bootable usb select '"UFD or USB disk drive"' then hit mo "f10" or save and exit it will boot to 1st boot settings which is yung Win xp na O.S., may lalabas yan na "'press any key to boot from cd"', hit mo lang kahit na anung key.
lahat po na step at nasa youtube boss search ka lang marami po tutorial don.