If you have a laptop (or a USB hard drive) sometimes you have stuff that you need to protect on the hard drive of your PC from prying eyes. TrueCrypt – Free Open-Source On-The-Fly Disk Encryption Software for Windows XP/2000/2003 is one possible answer to making sure that if you lose it at least people wont be able read your stuff . It uses Blowfish amongst other encryption schemes , Blowfish was designed in 1993 by Bruce Schneier as a fast, free alternative to existing encryption algorithms. Since then it has been analyzed considerably, and it is slowly gaining acceptance as a strong encryption algorithm. Blowfish is unpatented and license-free, and is available free for all uses. Have a look at TrueCrypt.
For a bit more detail on the subject have a look at Hard Disk Encryption – infoAnarchy Wiki
A Hard Disk Encryption programs run a memory-resident program is in the background, acting as an interpreter between the container file and the rest of the computer. Without the memory resident program and password, the file is indistiguishable from random data.