Mustafa,
I had absolutely no problem with Vista Ultimate Version. I'm in Information Technology field, and I've been part of Chevron's migration to Vista, Shell's migration to vista, so I know they are working good. Windows 7 is going to be a replacement of Vista.
Linus Torvalds wrote Linux and Linux is the most solid secure system in the market and it's open source. Free

I have worked on it, and I really liked it, specially their windows GUI (graphic user interface) flavors like Ubuntu and Kubuntu.