Writing about web page http://slax.linux-live.org/
First things first, SLAX is a Live distro of Linux. That means that it is an operating system which runs from the CD with no need to even have a hard drive. SLAX is the live “version” of slackware Linux and uses KDE. It's available from the URL above as an iso. For those of you who might not know how to use them but really want to give Microsoft the boot I'll give a brief explanation. “.iso” is a file format (it's a CD image), in order to use it you need to download it, then open it in a program that supports iso burning like Alcohol 120% (I think you might be able to get a free trial version of that somewhere) then you just need to go to the image burning wizard, open the file and tell it to burn to a CD. It sounds complicated but it's rather quite easy. The you simply need to put the CD in the CD drive of your computer, boot it up, go into the BIOS and then change the boot preference so that it automatically boots from CD before hard–drive.
When that stage is done (which is the hardest part of it all) you can boot up SLAX. Boot options are got to through F1, although unless you have any problems you probably won't need this too much. When I tried on my very old laptop I had to tell it to bypass hotplug, pcmcia and agp, on my new laptop it all worked perfectly. You might want to try using the boot options to load the CD to the RAM, I think this might make it work faster (as its loads easier to access than a CD); although I've not tried bench–marking anything. I think the boot option is something like “SLAX boot2ram” (it's in the list that comes up when you got to that section anyway).
When it's booted up to what you might call the main page there are some options there. You need to login (I don't know why) with name “root” and password “toor”, then you can use the “startx” command to open the more familiar looking environment, or you can have a look at “midnight commander” using “mc”... there are some other options which you can try at your leisure. When you use the “startx” command you get into KDE. This gives you a very pleasing set up, complete with nice graphics and pretty much everything that you would want as a home user (although unfortunately no pre–installed Open Office). The lack of an office suite could be seen as a bit of a problem; not too much considering AJAX office is getting up and running, to effectively work as a live office (see entry below on AJAX write). The GUI could easily be a replacement to windows, and even if you decide not to use it then all you have to do is take the CD out and you can go back to using whatever you did before. You don't need a hard–drive to use it, although obviously you'll need some form of non–volatile memory if you want to save anything permanently. I'd stick this on the Hard Drive. Browser is Konquerer. System seems stable; I've not had any problems on my laptop.
Of course it is far more secure than something like Windows with regard to viruses and spyware. Even if there was a virus written for this (which I'm not sure there is) you would need to download it, then run it, and even then you can fix the problem by simply switching it off and on again, because the OS is loaded each time anew. SLAX is also far smaller than your average OS, the iso is only about 187 MB, this means that it doesn't have all the bloat of bigger OSs and as such seems more stable and faster (there's less to go wrong). Using a hard drive simply as a way to store your information separate of the OS seems to work well. On the downside you have to extract the OS to the RAM each time, or load the CD up to a suitable point.
So would I recommend? Yes, it's free to download and you don't have to get rid of any other OS on your computer, so it is worth a try independently of whether or not your looking for a full time replacement. I would say it could easily be used as a full time replacement for another OS… which is what I might do.
Update: I ended up going with a somewhat regular OS, being Fedora core 5… SLAX is still worth trying as a way to decide if you like KDE, and if you were to boot it using a USB key into RAM then it would be really quick (still, it's best to have over 1 gig of RAM so that you can store enough data). I've also tried Damn Small Linux (it's so small – 50 meg!) it works really well on old laptops and has enough useful stuff to keep you working happily : )