Is it one you owned and operated? What is Windows Z?
I'd say go in and uninstall every program in Add/Remove Program that isn't essential. Go into user accounts and make a separate account for the kids with its own password (that you create, so you know what it is.) Do not make it an administrator account.
That will likely ensure that a lot of stored passwords and user account information isn't available on their user account. If you're on XP or older, that's not a guarantee, as Windows 7 stores a lot more data on a user account basis than older versions. Make sure to wipe the cache and cookies and installed web browsers, as that's a primary way websites remember your passwords, auto filling your search history, ect....
You could also revoke a lot of the privileges from their user account (because kids can inadvertently fuck up something by being inquisitive, and possibly mandate a reinstall) but that gets slightly more complicated.
Unfortunately, the contents of the hard drive are NOT stored on a user account basis, so, if bad things are in easy to find places, you'll have to deal with those directly. If they're buried in some nebulous part of the file system that kids are unlikely to find....
Smut is something you'll have to clean out yourself, and you'd probably know where it is. If it's someone else's smut, have fun finding it. I suppose you could use the Windows search bar in the Start menu and search for all .jpg, .gif, .png and .bmp files. That will show you at least where all images are located. The same would need to be done movie files, like .mpg, .avi, ect...
There's literally dozens of other things you could do to make a computer "kid safe" by turning on safe searching in your web browser, ect.... but I'm a little too tired to think through and list all of them at the moment.