I have a custom built PC that is running Windows XP. It has a dual core processor by Intel with 4GB of RAM. For my DAW i'm using Cubase. Kontakt works without a hitch on my system after installing it in the VST format. Kontakt uses a good deal of system resources, so people with less powerful computers beware. Expect to have a shortage of RAM with all of these samples to load! The program is pretty stable, it doesn't crash often.
I have been using kontakt for years. You can find it in a lot of different studios that do any kind of arranging. Its best assets are the ease of use and the customization that you can do on the different patches. The sounds themselves are kind of hit or miss. Some of the string patches sound realistic, while others sound like total 80's cheesy strings. As for strings, I have had good luck by combining more than one patch playing the same arrangement and bussing them to one reverb to improve realism. There is of course way more than just strings in Kontakt, but that has just been my main use for it. It's a good value for the price considering the wide variety of patches you get and the nice interface. I would get it again.