Hello. I recently bought a used laptop from my wife's friend and the laptop has the CentOS (Redhat Linux) installed but it originally came with Windows XP Home Edition. It asks for a username and password but thew owner doesn't remember neither one (lol). I was wondering how to I switch back to Windows XP or how do I go about changing the OS on this laptop? Thanks guys, I appreciate it.

-Mike-

If it's dual booted then choose Windows at the prompt and you could repartition having Windows reclaim the other Linux partition. If not, then repartition the drive and then reinstall Windows.

You will need partitioning software if you did not want to use a bootable Linux cd.

lord-fu is giving poor advice. Grab a Linux CD and install it. Format your hard drive, install Linux, and throw anything Microsoft related into a giant bonfire.
Kidding. If Windows floats your boat, that's your choice and you should follow lord-fu's instructions. Otherwise, grab something like Ubuntu or Fedora and install it (be sure to get the most recent [non-alpha/beta] versions).
That being said, I think Microsoft made a great stride forward with Office 2007. That is the only Microsoft product I have ever really liked.
But I say just install Linux over whatever's on there. [edit] line breaks aren't working for me for some reason...I've tried editing twice now...

How about a more un-biased approach? lol, jk.

Get GParted from www.distrowatch.com and delete you CentOS partition, expand the ntfs partition to fill it all up (assuming you want XP). If Grub was installed in the MBR, it will still be there when you reboot, just not CentOS.

As far as distros to use, there are hundreds to chose from. What is your experience with Linux?

Another option would be to just add a user for youtself.

In short, you would need to download a live cd of a linux distribution (perhaps Fedora).

Mount the partition that has CentOS on it, and edit /etc/shadow to remove root's password.

I don't know if this is making any sense to you (depends a bit on your level of linux experience) but if you need more help, I'm sure we can talk you through the process.

Step one would be to download/burn a live cd (http://fedoraproject.org/get-fedora would work)

Then come back

At the Grub screen (where it says "Booting Centos in N seconds), hit Enter. With the top CentOS line selected, hit "e", arrow to the "kernel" line and hit "e" again. At the end of the line, add " single" or " 1", the hit "Enter", then "b". This will boot you into single user mode. Once you're at a prompt, enter "passwd" to change the root password, then "reboot".

When the machine comes back up, log in as root with your new password, then go to System > Administration > Users & Groups and you can add/edit the users there to something that you know.