Windows XP Pro. The relability of the computer depends on the user. My system is very tweaked and optimized. Not even my sister, who has some kind of curse on electronical devices, can get virus on it.
Also, it has a lot of options to customize its looks and to scale it on weaker PC's, so It's my first choice on the PC's I sell too.
Oh, and I do believe in free software too.... ;)







