According to ComputerWorld, under Apple's longtime update policies, Macs running Snow Leopard will stop getting security updates this summer when Mountain Lion debuts. Snow Leopard was released in August 2009, just three years ago. In contrast, Microsoft will support Windows XP with security updates until April of 2014. XP was released in 2000, and the current service pack (SP3) is already more than four years old.
This is especially bad because almost half of all Macs are still running Snow Leopard. If there are Macs on your organization's network, it may be time to develop a plan for upgrading them to a newer version of OS X - or trading them in for Windows machines.