Nikhil
Dsve's link gives full details since there are so many ways of ending up with WIndows 8 on a computer but for a quick reply for typical home users here's an excerpt:
<< For a PC that has Windows 8 preinstalled by an OEM
OEM downgrade rights apply to only Windows 8 Pro and allow for downgrades for up to two earlier versions (to Windows 7 Professional and to Windows Vista Business). ..