TechGrunt has pointed you in the right direction. PGP or an equivalent program to encrypt the data is your best bet. Removing it is not a bad idea...but keep in mind, these are simple measures, just like locking your front door. A determined thief (with the right level of technical expertise) will go through your harddrive, looking in the pagefile, sector slackspace, etc. for left over fragments or temporary versions of the file. How realistic is this concern? ...I wouldn't lose any sleep over it; I just didn't want you to get a false sense of security. The nice thing about the more recent versions of PGP is it has a wipe utility that will securely delete the original version of your data, after encryption has taken place. It can also wipe any unused space on the partition.

As XTC46 and TechGrunt have said, removable media is your single best bet, but combine it with PGP or another form of encryption. There are many open source, shareware, or freeware programs out there. Just make sure you spend the time and energy to learn how they work and become comfortable with them. Use a strong passphrase to protect the encryption key. I have written a small white paper on passphrases, which you can review here.