I'm looking for the easiest way to encrypt an entire hard drive of a personal computer, and I still would like it to be quick. I'd appreciate any help.
The "quickest" (easiest?) way is to encrypt the partition during installation - if the disk is a complete Ubuntu system. Ubuntu comes with support for an encrypted /home directory and for encrypting the full system; encrypting the entire system would probably not be worth it. It would mean encrypting log files, system configuration, system binaries, and swap.
In any case, the use of ecryptfs would do what you want. Make sure you have ecryptfs-utils loaded.
If you aren't afraid of some technical documentation, you could try these excellent Ubuntu documents:
If installation is possible, you could try that - I think it requires LVM to use encryption.