Question: I recently reinstalled the OS (Windows XP Home) on a friend's heavily spyware/adware infected machine. It appears as though his kids (7 and 11 years I believe) were visiting sites that offered downloads for search bars, screensavers, and "cute" cartoony animations that anyone with safety in mind would never download.

Some of the concerns I noted on the machine were:

* The main account was Administrator
* Expired anti-virus software
* Lack of Windows updates
* Multiple firewalls installed

Basically, I'm looking for a guide for non tech-savvy users that would help them avoid getting infected again.

Answer: 
If the question is really to help them avoid getting infected again, and it is a computer for the kids, I recommend you to do some research on deploying [Windows SteadyState](http://www.microsoft.com/windows/products/winfamily/sharedaccess/default.mspx) for that particular computer.
The top rated advice here is all good, but there is no perfect solution. Not yet covered:

I would emphasize training above all else. Most spyware / adware is invited into the computer. Whatever your computer / OS & your religious beliefs about them, your web browser is your weakest point in the system. Surfing the web is analogous to driving randomly around town asking strangers if they want a ride. Keep in mind most people using Outlook have HTML mail enabled by default, effectively turning their email client into a web browser.

Consider using Firefox and installing [NoScript](https://addons.mozilla.org/addon/722). It's a solid plugin that by default prevents all scripts running. It's easy to add exceptions for sites you trust.

All the machines in my house were Windows NT / XP / Vista for years. I finally shifted to Ubuntu to simplify my life. I was tired of being the family systems administrator. I picked Ubuntu largely for these reasons:

1. No root account by default
2. No open ports by default
3. Huge software repository
4. Easy updates with a long support cycle
5. My favorite price. Free.