Windows ME - it self-destructed on me after a few months of usage
Internet Explorer 6 - gave me the best possible incentive to move to Firefox
Windows Genuine (Dis)Advantage - for me was really the final straw
Windows XP was highly successful though, although again they really failed to do anything very innovative with that OS other than keep it stable. By the end of 2006 I'd stopped using most of their products, having been a MS stalwart since Windows 3.1, and I havn't looked back since.