Re Windows XP. As is known Microsoft will cease to support this as from 8 April and won't release any further security updates.
For anyone with XP, if they are relying on Microsft Security Essentials as their antivrus program, Microsoft will continue to support this until 14 July 2015. Note that the Microsoft Security Essentials version 4.5 does not support XP.
Other antivirus programs will continue to support XP for a period of time but that period will vary between suppliers.
Although antivirus progams will help, if hackers discover a weakness in the XP operating system, there's no guarantee that antivirus programs will be able to prevent hackers attacking XP computers. Anyone sticking with XP has to accept that their computer will be at risk though if they disconnect from the internet, and don't use it for emails there shouldn't be a security risk.