Don't get a Mac. Ok? Don't. They are over-expensive, pretentious pieces of crap.
If you don't want Windows, install a free and open-source flavour of UNIX.
Personally I like Dell - I've always bought from them and I've always been happy with my PC. I like the way you can custom build a PC, but you can do this with a load of other brands as well, I believe.
As for Laptop vs Desktop - you need to think about what you'll use it for.
Do you want to do any wireless hacking? If yes, a laptop obviously - but make sure you look into the wireless card. You need one that can be run in promiscuous mode.
Do you want to game? If yes, a desktop may
be a better idea.
Do you want to use it for your music? Perhaps a desktop then for the superior hard-drive sizes.
Do you want to take it anywhere? Well then you're going to need a laptop, aren't you
etc etc, I could go on all day.
I use my laptop (an Asus Eee PC) as a main computer, I travel with it but I also use it at home (my parents are extremely tight on computer usage, they don't know I have a laptop). I use it as a server, for filesharing (with an external HD), for sorting out my ipod, for typing, for chatting, for watching movies and for surfing the web. This thing does everything I want and I'm very happy with it.
Choosing the right PC is very important - if you're a nerd like myself, you'll be spending a good bit of time on it, remember.