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: 
* Have non-tech users accounts be non-admin
* Teach safe browsing habits
* Never download anything that isn't from an 'official' source
* Use Firefox/Chrome/Safari/Opera instead of Internet Explorer
* Run malware scans regularly
* Set Windows to update automatically
* Install virus protection and scan automatically/regularly
* Install a firewall
* And again, smart browsing habits.

For firewall, either don't use one (and use Windows Firewall instead) or install one free one, like ZoneAlarm.

Edit: I see in your question you mentioned expired anti virus. Use AVG instead. It's free and will never expire and updates automatically.

Edit2: As others have mentioned, you could install software that basically locks the computer down. I have to disagree with this. I would much rather take the time and teach the users (especially the children) about how to properly use the computer to the fullest, rather than block them out.
Particularly if the main OS is Windows XP, I would recommend setting up a virtual machine ([VirtualBox](http://en.wikipedia.org/wiki/VirtualBox) is free and good) and putting a Linux distribution inside the VM. Do web browsing from Linux and run other programs from Windows.

You could also set up a dual boot - [Wubi](https://en.wikipedia.org/wiki/Wubi_(software)) is a dead simple way to install a Linux "virtual partition", but the VM makes it simpler to switch between operating systems.

While Windows Vista and Windows 7 can be very secure OSes, Windows XP has enough holes that trying to make it secure is very difficult.