Intel to release updated microcode going back at least 5 years

To fix the problems associated with the Spectre and Meltdown vulnerabilities, Intel will be releasing microcode [i.e. BIOS] updates going back quite far. How far? Initially they said going back to the 4th generation Core processors but now they are planning to go even farther. At least back as far as Core 2 Duo days. Updates will be available from OEMs and not Windows Update.

You can find the announcement here.

[Update 2018/03/05:] Microsoft will slowly be offering microcode update though Windows Update Catalog. One update will cover them all. If your CPU is not supported at the time, it will notify you.

The catch is that the update will not be through Windows Update but through the Windows Update Catalog. So you will have to manually download the update.

[Update 2018/03/16:] Microsoft added more CPUs (6th through 8th generation) now to the update. Updated 8th generation as well as the upcoming 9th generation CPUs will have the fix in addition to other protections, so Intel says.