If I'm not mistaken, XP is still the minimum we're trying to support. About a year or two ago we still had to worry about Win 2000, I'm not sure if we still need to cover that. I might have to leave this for @todb-r7 to decide.

Decision like this is sort of based on the market share:
http://www.netmarketshare.com/operating-system-market-share.aspx?qprid=10&qpcustomd=0