2.  Disconnect all peripherals (camera, printer, scanner, etc)
3.  Disconnect any ZIP drive
4.  Disconnect network/modem cable.
5.  Go to BIOS and set CDROM as boot device before hard drive.
6.  Insert Windows XP CD and reboot.
At "Press any key to boot to CDROM" quickly press a key and follow the prompts for a Clean installation:
Windows XP Clean Install (Interactive Setup)
If you are installing using Windows XP Upgrade, insert qualifying media in Step 4b.
Delete all partitions in step 5 of the above link if you desire to erase all data on the hard drive.
Partitioning and formatting will be done automatically.
All data will be destroyed, back-up important data first.

7.  Enable or install firewall BEFORE connecting network cable.
HOW TO: Enable or Disable Internet Connection Firewall in Windows XP

8.  Install Service Pack 2 first and by itself.
Reboot
The Service Pack will take care of most of the updates.
Then install ALL Critical Updates (if any) no more than 3 at a time.
Start/All Programs/Windows Update
Reboot between each group.

9.  After you are finished, you will need to load the appropriate drivers.

If your computer was shipped with Windows XP, these procedures may or may not apply.
Check your manual or contact the computer manufacturer for more details.

Make sure your computer is Safe and Secure when you are finished.