I'd like to point out the critical importance of protecting the "raw" WindowsXP installation from worm attacks, too.
This page shows how you can keep the computer out of trouble. If it's for a kid, then educate, educate, educate about avoiding warez, cracks, pr0n and other high-risk stuff that ends up letting Trojans, worms and viruses in the door, not to mention spyware/adware.
All that glitters is not gold...
Get him/her set up with
a Limited user account for normal daily stuff (web surfing, IM, email, etc) and a separate Administrator-class account that is only used when there's an actual need for Admin-level power (installing a new printer & its software, for example). The Limited account is a huge blow against accidental spyware infestations and other malware.
Also (duh) fully patch the system at Windows Update, and make sure the Automatic Updates feature is enabled and set to fully-automatic. If the computer has Microsoft Office, or any components of it such as Word or Outlook, then drag it through
Office Update as well.
If you can find out the model of Compaq that is being given away, I'll hunt for the drivers so you can have them on CD as well. A good order of installation is
1) Windows (keep the network cable unplugged)
2) install Service Pack 2 offline from a pre-burned CD (this enables the Windows Firewall)
3) Enable the Automatic Updates feature on the reboot after Service Pack 2 is installed
4) Now install your motherboard chipset drivers
before your video drivers, and reboot
5) Install the video drivers and reboot
6) Install the other drivers as the case may be (modem, audio, whatever)
7) Get the antivirus software installed and configured
8) Fully enable Data Execution Prevention
like this
9) Now plug in your network cable, update the antivirus software, reboot as needed, and get over to Windows Update for post-SP2 patches. Check again after rebooting, for more patches and for more antivirus updates, until it comes up clean.
10) Check the system over with Microsoft Baseline Security Analyzer (search Microsoft.com for MBSA)