Some 8 years ago I used to have a friend dropping by almost daily to surf (he didn't have his own laptop or PC) and I always had to clean up after him - I can't imagine how bad it would be if I had several kids all having their way on my PC. I'd be mortified!

Also, one other cheaper solution that people haven't really mentioned, is dual booting with hard drives that are separate and protected from each other meaning they can't infect your stuff and you can't read their infected stuff. No need for a new PC or laptop then and simply requires a reboot to pick the OS.

Administrator mode.

With the password being "stopbeingfuckingdumbassesalreadyyoupeople".

My brother in law had named his PC the PornMaster 9000 and it had wallpaper, screensaver and even some custom icons that proved it earned that name. He came to stay with us for about 2 weeks onetime and I was freaked out about him on my PC. But I checked cache and browser history and ran all my spyware checks and it looked like he behaved himself. Then a few months later the hard drive crashed and I used some recovery software to get the data back, and also recovered a few gigs of dirty movie files that had been downloaded and deleted.

...and I see nothing wrong as he cleaned up after he was finished ;)

On the other hand, my relatives are either tech-savvy or polite/smart enough not to install shit when they are around without my permission, so everything works fine. History cleaning I do before they come, on the other hand... :P

Oh, the horrors of having relatives, friends or fellow students that mess with your equipment...

The past : Windows 98 and no means of backup until i discovered Linux...
The Linux box was safe until i was away for some weeks and someone simply formatted the PC and put Windows 98 back on it, because i had not disabled booting from CD-ROM.
A BIOS start password ? They shorted out the onboard battery to reset the password.
This ended when I got Windows 2K.

Then came a girlfriend who had some trust issues with me "keeping my PC stuff" secret.
She took the PC to a friend and had him crack it and install surveillance stuff. I found the key logger and Backorifice some days later. When confronted, she was stupid enough to admit it. "You just can't trust a man to be faithful". I kicked that psycho bitch out.

Now, I literally lock away my box when not at home and log out when other people are around even for a pee break.

Yeah.

SOLUTION - don't let them touch your computer and what RedRagan said if you want to be nice.

You must be a really nice guy, letting a brother in law who named his PC "PornMaster 9000" use your computer.

For a minute there, I thought you were going to tell a story about how your brother in law makes sure his kids won't touch his PC :p . Having it filled with porn is a sure way of the wife/mother keeping an extra eye out so they don't touch it ;)

That was the right choice. Holy crap the stuff some people will put up with!

I also lost the majority of my "friends" back then because she made up fake stories. I have knife scars to proof who attacked whom but nobody took that seriously.
She sent me suicide threats. I answered with a cease-and-desist.
A little bit crazy is okay but never again such a psycho!

Addendum: In contrast see my current ex : she jilted me last week without any real hassle. We are even residing together in a shared flat. Problems will surely arise when one of us will have a new partner.

Have you checked your living space for hidden cameras and mics?

You never know...

Anyways, how do you guys manage to let such specimens enter your lives?

Haha! Yes, i did. Had a bug scanner and searched thoroughly. I was a bit paranoid by then.

She was pretty unsuspicious at first... the psycho behavior surfaced after a year.

Yeah and he deleted the files too.

Might want to double check, that guy in the corner can be hard to detect sometimes...

http://www.youtube.com/watch?v=UnI81iB0gBQ