Macintosh, that's the word <3
You don't need to worry about viruses, cause there isn't any :P
I use to load with Limewire in the past, but it was so sticky, that I changed.
Nowdays I have Vuze, which works sooo smoooooth on my MacBook.
Just be sure, that there is littlebit spare space on your hard disk. If it is too full, it will make your computer a bit slower.