You can encrypt any files or folders so they will only be useable when you are logged on by right clicking on the appropriate file/ folder and choosing advanced/ encrypt.
A quick search on windows XP help for "encrypt" shows the ins and outs of it:
"Encrypting File System overview
Encrypting File System (EFS) provides the core file encryption technology used to store encrypted files on NTFS file system volumes. Once you encrypt a file or folder, you work with the encrypted file or folder just as you do with any other files and folders.
Encryption is transparent to the user that encrypted the file. This means that you do not have to manually decrypt the encrypted file before you can use it. You can open and change the file as you normally do.
Using EFS is similar to using permissions on files and folders. Both methods can be used to restrict access to data. However, an intruder who gains unauthorized physical access to your encrypted files or folders will be prevented from reading them. If the intruder tries to open or copy your encrypted file or folder he receives an access denied message. Permissions on files and folders do not protect against unauthorized physical attacks.
You encrypt or decrypt a folder or file by setting the encryption property for folders and files just as you set any other attribute such as read-only, compressed, or hidden. If you encrypt a folder, all files and subfolders created in the encrypted folder are automatically encrypted. It is recommended that you encrypt at the folder level.
When you work with encrypted files and folders, keep in mind the following information:
Only files and folders on NTFS volumes can be encrypted. Because WebDAV works with NTFS, NTFS is required when encrypting files over WebDAV.
Files or folders that are compressed cannot also be encrypted. If the user marks a file or folder for encryption, that file or folder will be uncompressed.
Encrypted files can become decrypted if you copy or move the file to a volume that is not an NTFS volume.
Moving unencrypted files into an encrypted folder will automatically encrypt those files in the new folder. However, the reverse operation will not automatically decrypt files. Files must be explicitly decrypted"........ etc.
You do need an ntfs formatted hard drive, but it's something worth trying.