VMware distributes a driver[1] one can install to patch the microcode on Windows at runtime. If these updates need to be recognized before OS boot, this may not mitigate the dangers. But in my test lab, HWiNFO64 shows the current microcode version.

The Intel[2] and AMD[3] microcode can be downloaded but I can't vouch for how many Meltdown/Spectre mitigations these versions contain. The Intel one is dated 2017-11-17.

[1] https://labs.vmware.com/flings/vmware-cpu-microcode-update-d...

[2] https://downloadcenter.intel.com/download/27337/Linux-Proces...

[3] https://git.kernel.org/pub/scm/linux/kernel/git/firmware/lin...