Microsoft has announced it’s ending support of Windows 2000, Server 2000, XP w/ Service Pack 2, and Vista with no service packs installed. Vista support will cease April 13th and the rest on July 13th. This means that after those dates these operating systems won’t receive updates nor the more important security patches from Microsoft.
If you have XP with Service Pack 3 installed, your machine will still get updates/patches until April 2014 and Vista users with SP1/2 will have a support quite a while after that as well.
Of course another option would be to upgrade to Windows 7 which I’ll post my thoughts on in the near future.