[tdk]

It is entirely possible to trojan a system with an encrypted OS partition, by using a bootkit. For extra security, you should boot off a removable medium. Even this can be attacked if the BIOS is infected.

LibreCrypt is useful against very specific threats. Protecting against attackers that have repeated physical access to your PC, and the technical nous to install keyloggers etc. involves way more than just using a particular Windows program, no matter it's features.

[beagle3]

Indeed, note I said "easier" and pointed one of the easier ways enabled by lack of OS partition encryption.

I'll go even farther than parent: If your adversary is determined enough, you should assume that any physical access to your machine, for however short a period, means you should never ever use it again - and that you have no practical way to know if said access has indeed compromised your machine. See e.g., Thunderstrike.

corollary: You can never be sure that your machine, which has passed through 10 different hands (factory, tester, packages, store, courier, ...) is not trojaned to begin with.