I think Windows XP is the most reliable OS. If one is familiar with XP, it will be easier to use other OS like Vista and Windows 7. It's like as if the technology used in Windows XP is the foundation of newer operating systems. We still keep our Windows XP OS on our desktop at home. But I had to use Vista since it came preloaded in my laptop. Though I've been using Vista more, I still think that XP is more reliable than Vista.
I`ve been using my Windows XP for two years without any problems. May be NOD 32 antivirus helped me in that.