How can I format a laptop having two operating systems on it?

Answer 1

If it came with a restore disk simply run the disk it will return the computer to "Factory New" condition

If not get a freeware program like Boot and Nuke

Here: http://www.download.com/Darik-s-Boot-and-Nuke/3000-2092_4-10165154.html?tag=lst-0-1

Answer 2

You can use kill disk http://www.killdisk.com/downloadfree.htm
you have to download the free file and the iso and burn to a disk. It tells you the partitions and you can erase the partions and format the drive. or use a windows xp disk