The good news: Shortly after its initial release, Microsoft suspended shipping its Spectre and Meltdown Windows patches to owners of AMD systems after some users found that it left their systems unbootable. Microsoft partially lifted the restriction last week, sending the update to newer AMD systems but still leaving the oldest machines unpatched.
Now the company has an update that works on those systems too. If you're unfortunate enough to have installed the previous, bad update and now have a system that crashes on startup, you'll still have to roll back the bad update before you can install the new one. We've read reports that this is indeed possible, but unfortunately, Microsoft only offers generic guidance on troubleshooting blue screen of death crashes, not any specific steps to fix this specific issue.
The bad news: Intel has previously warned that the microcode update it issued to provide some processor-based mitigation for some kinds of Spectre attack was causing machines with Haswell and Broadwell processors to reboot. It turns out that the problems are more widespread than previously reported: the chip company is now saying that Ivy Bridge, Sandy Bridge, Skylake, and Kaby Lake systems are affected too.