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: 
Best solution I found: gave my kids a Mac mini with parental controls on... No virus problems, they can't really install any crapware even if their friends point them to some. Also, game choice is limited, which is a big plus. With that, they spend much more time exploring the web, experimenting with making music or videos than playing games.

Plus, the mini makes 0 noise and fits anywhere, it's perfect for a bedroom.
Kids love it too, don't miss games much, they prefer to play on the Wii anyway...

So, my advice: forget XP, it's a lost cause. There are much better things out there.
If you want to reuse you machine and don't want to buy a new one, install Linux on it. The desktop effects on the new Ubuntu are amazing (see <http://www.youtube.com/watch?v=dlhD_4pK4MM> for example), kids always get excited with that. Plus they'll have tons of things to explore with Linux, much more interesting than old grandpa's Windows XP clunker :)
With linux, you're basically also virtually guaranteed to remain crapware-free.

I'm moving away slowly from Windows (started late last year), and I am very glad I did so far, I refuse to waste any more time fixing Windows installs.
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.