I have four XP computers at home. One is a desktop that my wife uses and I'll run it into the ground, from a software perspective. With more memory, it's got enough power to run a newer OS. If need by I'll installa new hard drive with a new OS and keepthe old one as an external. The drive has almost not data. It's all external.
There are teststations out there that still run Win2k and even Win98. And probably a few running DOS.
I've still got 5 or 6 XP computers at home as well. All currently in applications with no need to connect to the internet. Actually one is currently unplugged waiting for me to get around to taking it somewhere to get recycled...
One of the XP computers I have is a dedicated lab computer. It seems that Windows 7 doesn't really like parallel ports so I'm still running XP on that computer so I can run my prom programmer with the parallel port interface (and DOS software). Not that I use th eprom programmer much any more but I'd have an immediate use for it if i couldn't run it any more.
A month or so ago I finally got rid of the really old computers that couldn't even run XP...
I've found that Windows is reasonably reliable if you never, ever connect the PC to the Internet. I've a Windows 2000 machine on which I develop code that I want to be able to run on (pretty much) any Windows machine, and a Windows 7 machine I'll use if someone pays me to do it.
Otherwise, it's all GNU/Linux.
"Why Windows 2000?", I hear you cry. Well, because I rather like Windows 2000 and dislike, hate, abhor, or loathe the others depending on the version :-) I rather liked Macintosh OS before it went to 7 and wasn't fun any more.
While it may be expensive to upgrade, the downtime that may occur due to viral intrusions or component failures is far more expensive. As indicated in our article, we recommend migrating to Windows 7, because it is the most straight forward migration path for Windows XP applications. You should also buy a new computer or PXI system that will provide a migration path to newer/faster hardware buses such as USB 3.0, PCIexpress, PXIexpress, etc. Even if you don't need the performance, consider that most new hardware is being designed for these newer buses. Upgrading the hardware helps improve maintenance.