Microsoft released a security update on January 3, 2018 to insure compatibility of a Windows updates related to CPU security flaw with anti-virus software products (see Microsoft Security Bulletin KB4072699
Fortinet tested the latest active FortiClient software versions 5.4.4 and 5.6.3 and found them fully compatible with Microsoft's January 2018 Security Update.
It safe to use these versions with the security update.
Microsoft requires that the following registry key exist on all
compatible systems, even if there is no AV product installed:
the latest active FortiClient versions 5.4.4 and 5.6.3, it will be
required to add this key manually, or using Group Policy, in order to receive
the January 3, 2018 security update.
Fortinet will release new versions of FortiClient prior to January 9, which will add the required registry key automatically.