I’m a big fan and regular user of TrueCrypt, a completely free and open source program that creates virtual encrypted disks on Windows and Linux volumes. Like a lot of other such programs, however, mounting the encrypted volumes can get a bit tedious.
AllThingsMarked.Com has a nice tutorial on how to create a batch script to make it a lot easier to mount and unmount a TrueCrypt volume.
The one defect of the AllThingsMarked.Com approach is that it doesn’t mask the password during entry, but Peter Hesse has a slightly modified version of the AllThingsMarked.Com method that does mask the password as long as you’re running Windows XP or Windows Server 2003 or later.