If you are buying a fresh ( box) piece, make sure that you've opened it and made them complete the OS ( windows )  installation there itself.

Because, after buying if you come across any issues they wont take the responsibility, but will tell you to go to their supplier.

Finding the place.. running after it... are soo much hazzles...  

Most of the laptops comes with OS pre-installed and will take only some half an hour to complete the installation process and first boot.