You are going to have to get into BIOS Setup on your computer to set the CD/DVD drive as the first bootable device, so you can then boot your XP CD, unless it provides the ability to press a Fn key at boot time to bring up the boot selection menu. If it does, use that.
How you do this varies with each manufacturer. Normally you press F1, F2, F10 or Del when you see the manufacturers logo. Often there is a clue somewhere on the screen when you boot normally telling you what key to press. Some computers (most newer ones) have a key you can press (often F12) to bring up the Boot Menu from where you can select the CD/DVD drive to boot from.
If you don't have an XP CD, download and run ARCDC by Artellos
. It will download the required files from the Microsoft Web site and create a .iso file. It will also offer to burn the iso image to a CD for you. If you don't do it then, there is a good freeware burner called ImgBurn
which will do it at a later time. If you're unsure how to do this, you can follow the tutorial here: How to Burn an ISO File
. This can be done from any computer.
Boot the CD and at the first opportunity, enter R
to start the Recovery Console.
Next, choose which XP installation you want to login to. Normally this will be 1.
Lastly you will be asked for the Administrator password. Just press the Enter key as the Administrator does not have a password (unless you gave it one, then you would enter it).
From the command prompt, enter: chkdsk /r
Allow it to complete undisturbed. It can take an hour or more depending on the size of the hard drive.
Remove the CD, type exit and the computer will reboot. Hopefully that will have fixed the problem (corrupted file system).