A:About 5 years, assuming you have Windows XP now. Microsoft will support WinXP and supply security patches for it until 2012, IIRC. Actually they often keep supporting operation systems for more than 5 years after they stop selling them, I think the support for Windows 98 ended in 2006.
However, if you're a gamer, you may need it for new games with DirectX10, and this could happen in 2-3 years.