to make it legal, you would have to make sure that you are not breaking the law.
As far as an anti-M$ site for example this means
a) be prepared to go to court for anything, this means you should be able to prove something is true, if you say it is, or prove something is false if you say it is. There is no point going to court over a comment like "Bill Gates has sex with small animals", unless you have the photos.
b) don't violate any agreement you have signed. This means, if you have ever used a Microsoft product, go and read the LATEST copy of the relevant agreement. Remember, at the moment Microsoft is able to retroactively change the contents of their user agreements so that the document you signed is no longer the one you are bound to (this is because the document you signed gives them permission to shift the goalposts whenever they like).
You might think this is unfair, but it IS LEGAL. For the time being anyway.
point b) is the hardest with regard to Microsoft or any company with ingenious little clauses like that in their licensing.
