I have a Windows XP desktop. (Years ago it became my secondary PC, now it is thirdish.) I still use it frequently. Way too sentimental not to. (Using it now actually!) (XP was the OS I knew, and still know, absolutely inside and out 100% until fairly recently with Windows 10 as well)
Since 2003 it has been off less than
100 total hours
max. It's probably closer to around 40. (I know I remember replacing the power supply once, it was off around 12 hours then because I was too tired to do it until the next day.) It has always been plugged into a UPS. On an average year it is only off for minutes, which would be during a few reboots or furniture moves.
The current uptime is 2819 hours (117 days). I'm fairly certain I had it on for over a year before, and am sure I've been in the 200s (it was in the 200s the one before this one).
And for a couple years it was folding proteins 24/7, running "flat-out" as Blue Bubble says.
I have used it online frequently since I bought it. Still to this day. Never had virus protection unless you count noscripts and adblockers. Just don't click on danger. It's that easy. (Being in IT helps to know what is danger though.

) Though I would scan with Kaspersky maybe twice a year, rarely would it find anything, no even a false positive.
As far as I remember I only got malware on this PC two times, easily removed with a Kaspersky scan both times. One was an Adobe Flash exploit, kind of hard to avoid that one. Knew instantly as soon as it ran what had happened.
I have never noticed any problems after the supposed end of XP support (they still did many security updates for years afterwards). I still get the "Support is Ending in April" (2014) pop-up every month. I'm too sentimental to check the "never show this again" box, and it (KB2934207) apparently was not coded to stop appearing after April 2014!
I have never ran a registry cleaner on it either. I've installed and uninstalled countless programs on this machine in the last 17 years. I have a feeling thatit might have the most bloated XP registry in the world.