If it ain't broke, don't fix it...and anything worth doing is worth doing well. If my XP machines are running fine, I'm going to leave them alone. And if a new OS is worth running, it's worth more on new hardware.
I pretty much have a policy where a new OS = a new PC, one in which the hardware is intended to support that particular OS. And as I can't afford one of those right now, I have to stick with what I have.
Only once did I break that rule, with an old snail PC, and only because I really hated Win98 SE. (WinME was such an improvement in stability, once I worked out a quirky driver issue)
The last PC I purchased, I took advantage of an XP downgrade option, rather than using Vista. And with the additional hardware I recycled and put into that box, I am glad I did. There are no drivers beyond XP for the controller card I installed into it. Upgrading it would mean losing access to the IDE hard drives and the only burner that machine has.
I am also not convinced that all my software would make the transition to a newer version of Windows, smoothly (sorry but XP compatibility mode probably won't cut it for some stuff that was written before XP was released, which already suffer some buggy glitches running on XP). If and when I do decide to buy a new computer with a newer version of Windows, it is very likely I will end up running XP in a VM for a few things I can't live without. (if it wasn't for those few apps and my extreme love for the Windows taskbar, I'd probably make a 100% transition to Linux)