I'm using 2 computers, my main computer (running Vista) and my lab/guest computer (running XP). They're both on simultaneously, but most of the time I'm the only one using them (they are in separate rooms, so I don't use them at the same time). Occasionally I let a friend use my lab computer for whatever they need to do on the net while sitting at my own computer.
@Stardock; I suppose in a strict sense, this violates the EULA, and I might get in trouble for writing this post, but I really think it's fair use. I'm not going to write a sobstory here, but I'm not rich, and I've just recently managed to get my computers entirely converted to legally licenced software. If you consider my use of the licence bad in any way, just notify me about it, and I'll sort it promptly.
On a side note; EULAs are void in Sweden, one of the good things about our legal system. It's not in any way considered a valid legal document, as that requires a handwritten signature by me. And even with that, my legal rights can't be signed away.
Personally I live by a single law; Common sense. I really don't bother myself with what's legal and not, I do as I see fit and feel is morally right. I've never run into any kinds of trouble with that.