I'm using PGPDisk for all my drives (without the C: drive, with the OS).
You create a single encrypted file on the clean harddisk and everytime you
want to access the encrypted files in it, you have to enter a passphrase.
Then all what's inside is mounted and accessible as a new drive.
Another program that should do something like that is Drivecrypt.
I heard it can also encrypt the system drive...
Or you use a hardware tool. Abit has something like that (Secure IDE).
You have to insert a key (I think it's an USB-stick) in to the IDE card then
you can access all. But if someone gets the stick too you may get
problems...
The best choice is using something like PGPdisk and then install a program
like VMWare. Then you can install a second OS (Win98, Win2000, Linux
and so on) on the encrypted drive. Then really all is hidden
All the best,
Calahan