Update 7/16/2012: Microsoft has extended support of Windows XP to 2014, which means patches to the operating system will be available for another couple of years.
Of course many businesses have long since migrated to Windows 7, which is a mature, stable, Operating System.
The latest word we have from our vendors is that Microsoft is extending the availability of XP until June of 2010.
Vista seems to have received a lot of bad press and the market pressure on Microsoft to continue to make XP available must have been significant. For those that don’t run vertical market apps that might still be a bit wobbly on Vista, we think Vista is an excellent choice. There is widespread support for it nowadays and device drivers are no longer the issue they once were.