I am actually trying to reinstall windows to a firend's laptop because it was full of viruses. He did not have a boot up disc so I downloaded one from the internet. I want to completely erase the disc but when I went through the process, it just installed another XP on there. So when I log on, it asks me which XP to log on.
Can someone guide me to reinstall windows and make the laptop like new again?
My title is misleading. It is a friend's PC so I am guessing that it has not been formatted. How do I format the C Drive? Just reight click and format?

his laptop contains two Operating System. It can be done if a PC or Laptop contains two or more partition or harddisks.

if you want to completely erase the disk, you have to delete the partition on it, and set another if you want. WIndows XP installation can guide you through the process, just spent some time reading what is displayed on each page.