Lord Evermore is correct, but lacking just a bit if you are not familiar with doing an installation repair in XP. This is the detail he is missing . . .
1. Boot from the installation CD (being prepared to hit F6 and supply the SATA floppy if you are using that type of HDD in the new configuration).
2. On the first screen, hit ENTER to start an installation. The second choice sounds like repair, but that is to take you to the Repair Console, which is not what you want. This is for doing simple little command-line repairs.
3. Screen next will be the EULA, and you hit F8 to accept it.
4. Screen next will notify you that you already have XP installed and give you the choice of hitting "R" to repair it, or ENTER to do a fresh install.
5. At this point, hit "R" and from there on in it looks just like a fresh installation.
6. During the final reboot, it may ask you to enter usernames. Leave them blank and go to the next step so that it will reuse your original user setup.
7. You will have to re-activate the operating system, and depending on how many things have changed it may require you to do it by phone.
8. In many cases, the automated phone process will work, but in the case that they say they can't activate it and transfer you to a human, just tell him that you fried your system and the motherboard, memory, and cpu had to be replaced. This is a legitimate reason under all licenses, whereas telling them that this is a major system upgrade may violate it depending on which version you have.