To expand briefly on this story of Leon’s about the problems with security in Internet Explorer I will add a quote from the source article:

Actually there was only one period in 2004 when there were no publicly known remote code execution bugs – between the 12th and the 19th of October – 7 days in total.

So for 359 of the 366 days of 2004 (it was a leap year) users of Internet Explorer were vulnerable to being hacked..
I await their figures for 2005 with interest!