CRM and Internet Explorer 11
Many users now have Internet Explorer 11 because of updates via Windows Update.
However, as of writing, according to this article http://support.microsoft.com/kb/2669061 CRM does not support Internet Explorer 11. I would be very surprised if support for IE11 does not appear in Jan or Feb 2014.
May 2014: CRM 2013 supports IE 11 with Update Rollup 2 and CRM 2011 supports IE11 with Update Rollup 17.