You can still use XP machines, my 9 year old laptop is also XP while my desktop harddrives all have windows 7 ultimate installed on them.
A friend that does computers for a living said don't use internet explorer to go online in the future with XP. Download and install free Mozilla Firefox and use it for your XP internet browser if you havn't already. It's much better and safer.
The hackers out there are going to be looking for XP machines using internet explorer which is going to become more vulnerable then it already is and they will be hacking into those computers more and more in the future.
He also said the pay versions of antivirus like avast which are better and safer than the free versions and that will help XP if you use it online.
In the end, win7 is the best microsoft OS, most people know that, and MS will keep supporting it for a while.
You can also switch to a linux OS. It's open source and free.
https://www.google.com/#q=linux+OS