James:
JN is absolutely right about TrueCrypt, it’s an excellent tool. Be aware, however, that you can be compelled to disclose your encryption keys in the UK legally [1], and you can always be compelled to do so via extra-legal means. If you have any data that you truly wish to keep secret, a good start is to use a second TrueCrypt volume containing important data inside the primary volume which contains
data that is less crucial. Regards, – PH