This thread probably has more to do with copyright issues than with KaZaA... When it comes to P2P networks, it depends upon what you are sharing/downloading, not whether the network/software you are using is legal/illegal.
The real issue is one of copyright law and for the most part, many people do not understand it (I do, but mostly because I was involved in publishing for about a dozen years.
Basically, most "western" countries have similar laws, thus the dutch law cited earlier pertains to material that is gathered for an academic use (i.e., preparing new material for publication/education). In almost all cases, such material will be cited in a Bibliography of the end-result, which will also be copyrighted. In the United States, this is also known as "fair use" and is very limited in the amount of material that can be copied and used.
The basics: Copyright is created at the time of creation... that is, if I hand-write a manuscript of a story, that story carries a copyright as of the time that I hand-write it on paper. The same is true of using a computer and word-processing program. As I edit/change my original work, a new copyright is created with each change. When a publisher buys and publishes the work, it carries a new copyright on it because the work has been edited (changed) in some manner. That's why most editors don't pay any attention to a copyright notice added by an author (it shows paranoia, more than anything else). The only time it is necessary is in a collection, such as a collection of previously-published short stories.
Take a look at a CD of "greatest hits" the next time you are in a store... In many cases, you'll find that each track carries its own copyright, dating back to the original recording.
Copyrights last a long time and apply whether there is a copyright notice or not. So, if something was produced commercially (and it doesn't matter what it is, it probably was) and was not placed in the public domain, then it is likely copyrighted. This includes snippets, sound bites, scenes, photographs, clip art, and any of a dozen other creations. Very little of what you find on the interest is public domain, despite anyone's feelings that the material is posted and therefore, it should be free... Copyright law simply does not work that way.
Who has what right to do what? A lot depends upon how you purchased the material. Generally speaking you do not have any right to copy anything, except under the "fair use" provisions of the law. You might as well visit a library if that is the case. Even if you purchase something, such as a commercial CD or DVD, you do not necessarily have a legal right to make a copy, even for backup purposes. This isn't to say that you can't do it, it largely depends upon the conditions of the sale...
Which brings us to licenses. Licenses are conditions of permission to do something. In the case of software, the license spells out the conditions under which you may use the software. You have not purchased the software, only the right to use it! You do not own that software. Ownership explicity implies that you can do whatever you want with the software. That is just not the case.
The same is true of any Intellectual Property or creative work of art. You may purchase a copy of such a work, but you do not own the work itself (unless you have purchased the original and the purchase agreement specifically transfers the ownership (copyright) to you. This is what the current squabble over UNIX is all about. The SCO Group claims they own UNIX... which is a long story in and of itself. Novell, the previous "owner" says "no, you don't" and has a court case against The SCO Group to show that they did not expressly convey copyright to TSG. And, the way copyright law is written, it must be convey specifically in writing, not something conveyed through any general terms of a contract sale.
So, what this is all about is that very few people, except those who actually own the rights under contract or by virtue that they are the creating artist, have any legal right to provide, offer, or share any copyrighted work.
Sharing and copying of commercial and/or copyrighted works is illegal.