So, you've been directed to perform a "repair" installation of Windows. First of all, let us see what's different between a repair installation and a full one. You don't format your drive during a repair install. Any documents that you have are not deleted. Any programs that you installed will still be on your computer. Thus, it's a perfect solution for the cases when your system starts to behave "strange". There can be hundreds of possible causes: numerous error messages, inability to boot in normal mode, inability to use some windows features, such as back-up or system restore. A repair installation will fix them likely.

The first and the only preparation step is to get your Windows XP installation CD and insert it into CD/DVD-drive of your computer. You may also need to configure your computer to boot from CD. In order to do this, open BIOS configuration setup at computer startup (it's usually accessed by DEL or F2 keys, check your computer's screen at startup, there usually is a sign saying "Press DEL to enter setup" or something similar).

Once your system recognizes a CD in the drive, you will be prompted to "press any key to boot from CD". Here we go:

Just wait while installation program examines your PC's configuration and loads all the necessary files.

Here is the first dialog. You should press ENTER (not "R", as some people do by mistake when they see "repair" word and don't pay attention to "recovery console" after):

Then you have to accept End User License Agreement...

And proceed to selection of partition to install.

You should see your old installation of Windows, it should be selected by default. Now press "R" to begin repair process.

Now it's goes almost the same way as normal setup. Examining drives, copying files, configuring settings and so on. After it finishes, your computer will restart. Let it boot from hard drive again and let your system configure display settings automatically.

That's it, we're done with repair installation.
Congratulations.