Time to look at the other side and see if there is anything new and horrible waiting under there.
Aside from the charred area of interest about half way along the left edge, there does not appear to be anything else obviously wrong with it. I did a quick check on all the diodes around the auxiliary transformer just in case and did not find a shorted or open diode. Even the zener responsible for scorching the PCB still checked out fine. I went through the extra trouble of pulling it out and hooking it up to 24V through a 100Ω resistor and got 18.4V, which means something else must have failed first.
The AR300's main outputs are driven by an UC3845 controller in SOIC-8 packaging, while the APFC function is managed by a PCS01. To cram all the electronics and a 120mm fan in about two thirds the volume of a conventional power supply, the majority of low-power resistors that would have been through-hole back then have been surface-mounted on the bottom. Ceramic capacitors and some small diodes have also gone SMD. While this is often taken for granted in modern supplies and more complex circuit boards out of necessity, it was not as common a decade ago when pressure to build more compact units was not so high.
Taking a closer look at that blackened area under the zener diode, it got so hot that the glue between its copper pads and the circuit board burned off, leaving the copper pads and part of the traces leading to them loose. For this to happen, the temperature had to significantly exceed 130 ºC for some time. I could reattach the pads to the circuit board using high temperature epoxy, but before doing that, I would need to lift the pads, scrape off the charred board area and the bottom of the pads to get reasonably clean surfaces for the epoxy to bond to, then press the pads to the board until the epoxy cures. I doubt the abused traces would survive the bending they might get subjected to during the cleaning process, and a $10 tube of epoxy is rather expensive for a likely one-time use before it expires. So, I will settle for re-soldering the diode with the diode and pads pressed snug against the board to remove as much slack as possible. Securing and sealing everything in place with superglue might not be a bad idea either, assuming the zener does not normally get anywhere near as hot as it had to be to cause this damage.
The large pad on the left connects to the short-leaded end of ZD4 and got so hot that the solder mask on part of its trace sputtered off, leaving the reddish-brown copper underneath exposed. The long lead of ZD4 connects to the other large pad and only shows moderate discoloration. How hot does a pad need to get to make the solder mask not merely darken but actually smolder clean off? I could not find numbers, but I suspect we are talking somewhere north of 200 ºC, possibly hot enough to melt lead-free solder. Now that I think about it, I remember smelling overheated electronics for a few days early last year and not finding what it may have been. I did not think of trying to turn on my P4 as a probable source of the smell since I had no need for it. This must have been it.