I still can't believe how long it's been since XP first came out, and that Microsoft has been supporting it for that long. Still, it had a good run and it was a very stable operating system even to this day. If you are going to upgrade, then I would suggest going with Windows 7. Windows Vista was full of bugs and Windows 8 is terrible.
There's also Windows 9 coming out which is said to happen in April of next year. Here's hoping Microsoft makes it more like a regular desktop computer and gives it the stability of Windows XP.