This is an introduction video for begginers, if you have never seen or heard of PGP Desktop, this video explains briefly in 5 minutes what it can do for your privacy.
If you want to keep all your computer data encrypted, including the OS, after Truecrypt and Drivecrypt Plus Pack , PGP Desktop would be my third choice. As far as I know all three of them are uncrackable, even by the highest powers out there. But bear in mind that it is illegal in the United Kingdom not to reveal your password to your encrypted files when requested by the authorities.
PGP 9 will not protect you against a Government forcing you to give away the password for your personal secrets, you should be using Truecrypt or Drivecrypt Plus Pack if you think this may happen to you. Both of those encryption schemes provide for a hidden operating system which existence can not be proven, you can not be asked to provide what can not be proven to exist.
If you are comparing encryption software you can read my past review of Drivecrypt Plus Pack v3.94