Set the BIOS to boot from the CD drive first. Next, back up any data that you need to keep. Put the Windows XP CD in the cd drive and restart the computer. Delete the partition and decide on a new partition (go with maximum size). FULL (long, not short) reformat the hard drive with the NTFS format. It should run automatically. It may take a couple of hours to complete. When prompted, enter the CD key serial number located on the Windows CD case.
When it is done and you are on the desktop, defragment the hard drive. Then you will need to re-install the following drivers and software programs:
1. Chipset drivers
2. Video Card Drivers.
3. NIC/Modem Drivers and ISP software
4. Windows Update, High Priority Updates
5. Windows Update, Optional hardware updates
6. Windows Update, Optional software updates (don't load unnecessary items)
7. Other Drivers (try to automatically update first)
8. Major applications. Fully update each application before installing the next.
9. The rest of your apps. (run disk cleanup)
10. Anti-Virus software and update it.
11. Configure email
Set Windows XP settings the way your sister wants them (the screen resolution and refresh rate, folders, icons, desktop picture, etc.)
Afterwards you may want to defragment the hard drive again and install any other programs you want to have like Windows XP Service Pack 2 from microsoft.com
If all goes well, enjoy your computer.