Use some "live" CD to boot. Backup any data from the main drive, the one containing the windows folder. Use an external drive for that. If you want to use some CD or DVD, you'll probably need several DVDs, it's easy to have over 100 Gb of data to save...
If you can't access your main drive: Install it in another computer to attempt to access it's content.
If you can't access it after booting from a live CD AND still can't after installed it in another computer, that drive is dead and all it's content is lost.
Using some torrent application, find and download a Windows XP installation CD image. Burn it to a CD-R.
Boot from that CD. During the initiation, you'll be asked what you want to do: Chose the "Repair" option.
That option will use your actual product key. If it can't find your key, you'll need to enter a valid key. You should have a sticker on the computer case with that number.
The installation will proceede normaly.
If your original drive is dead, do a clean install on a new drive. Get an ATA drive (use those gray ribon cables), your XP computer probably can't use the SATA ones (use a thin connector with a single plug at each ends).
Create a recovery disk and store it in a secure place. It may be a good idea to make a new one periodicaly, like once in 1 or 2 years.
Please note that this "Repair" process is NOT safe for any data cituated on the main drive.
Any data on a secondary drive, or on another partition of the main drive are safe. You also will need to reinstall all your drivers and most applications.
Once your data are recovered, you may need to retake ownership of many files.