0 Viruses on any of them.
I audibly chuckle when people use this as an excuse to keep XP.
Let me explain something to everyone: XP IS NOT SAFE! You're not vulnerable to more viruses, per se. You are vulnerable to EVERYTHING. XP does not support TLS. All versions of SSL are now considered unsafe and should not be used. By virtue of the fact that you're using XP, ANYTHING you think you are doing "securely" online, is not. The SSL cipher suite is dead and completely vulnerable.
XP now has numerous zero-day exploits available for anyone to use on the Internet. This means that if someone wants to get into your XP machine, THEY CAN. They don't need a virus or even for you to do anything. If you're browsing your bank site or trying to shop online, every last bit and byte can be read by a third party. This is not hyperbole, this is truth. I've seen it done at several black hat confabs and in person in my own company.
The only way XP could be considered even remotely safe is if it is completely and totally disconnected from the Internet and used as a terminal of some type or for playing old games. If you are using XP on the Internet, you might as well walk through a mall completely naked with your credit card numbers and SSN written across your entire body. There's nothing you can do to fortify a Windows XP machine. It's 100% unsafe to use on the Internet. Caveat emptor.
I am sure the Chinese and Russians are happy with the decision of the Navy.